are there any non-biased, independant sources outside the Torah and Judeism that can confirm that 3 million people witnessed all that?
No, because no one else was there other than the 3 million witnesses who you say do not count. There were no reporters at the scene, no cameras. Just witnesses, like most of history which is told from witness stand point.
is there any documentation of their testimony?
It was told to their kids and then to their kids and so on. Not what you would call documentation.
Let's pretend you're G-d and you want to transmit a religion to a people. You have two choices: You can either give it to a messenger to give to the people, or you can give it to the entire people directly. Which is the better choice?
The answer is found in this poignant story about an Indian chief:
One day the Indian chief dies, leaving three sons and no will. No one knows who's supposed to become the new chief and all the sons want the position.
The first son says, "I should be the new chief, because I'm the oldest." The second one says, "I'm the strongest warrior, so I should be the new chief." The third son says, " I'm the best choice because I'm the smartest."
There's a lot of campaigning going on and more than a few internal battles, until one day, the second son comes running into the camp, shouting, "Stop! I have the answer! I know who should be the new chief."
Everybody says, "What happened? How do you know?"
"Last night," the second son says, "my father came to me in a dream and told me I'm the new chief!"
Everybody starts dancing and celebrating and congratulating the second son. Until one old man raises his hand calmly and says, "Why should you be believed? If your father really wanted us to accept you as the new chief, he wouldn't have come to you in a dream - he would have come to us."
This story illustrates a crucial concept: If you want a revelation to be accepted by everyone, it's obvious that you would come to all the people, rather than to one person. That's clearly the most effective way to avoid any doubt. Why? Because I can make up stories about myself - and if you like me or trust me, you could choose to believe me. But if I make up a story and say it happened to you, then there's no way you'll believe me unless it really happened.
It's obvious that you can't get away with a lie on the basis of someone else's experience.
So if you're going to start a religion and you want to make sure everyone's going to accept it, the intelligent choice is to tell everyone, not just one person.
If it is true, then everyone in the national group will know it at the deepest level of knowledge, since everybody in the group was actually there. There will obviously be no need to present any additional evidence to anyone of that generation.
Also, the next generation will know that the event occurred, both because their own parents who were direct eyewitnesses told them, and because everyone else in the nation is either a direct eyewitness or the offspring of a direct eyewitness.
What if a large section of the nation were somehow duped - and were convinced to the extent that they actually passed on the lie to their children as if it was their own personal experience? This would not yield a believable, communicable, verifiable national truth, because the next generation would find many amongst them who either denied the universal character of the national claim or were never told about it by their parents.
Of the 15,000 known religions in recorded human history, how many stake the foundation of their belief on the idea that G-d spoke to their entire nation?
One. Judaism.
Isn't that strange? If a national revelation is the best way to go, why has no other nation ever tried it?
The answer is that this is one lie you can never get away with.
Let's take George Washington as an example. The fact that George "chopped down the cherry tree" is legend - it may or may not be true, but we'll never know. Though the fact that George Washington was the first president of the United States is verifiable historical fact. Why? Because there were many eyewitnesses.
Now let's apply this to religion.
If someone claims "G-d spoke to me," then other people have a choice to believe the claim or not. Some people will choose to believe the claim - and from there could start a whole new religion.
But if someone claims that "G-d spoke to all of you," he'll never get away with that if it didn't really happen. Because if an event never happened to someone, you surely cannot convince him that it happened to him!
And that's exactly why no other religion in history has ever made the claim of national revelation.
The point is not to denigrate other religions; the point is merely to strengthen our belief in Judaism. So it's worthwhile to investigate just how many people claimed to see Jesus rise from the dead (about a dozen), or how many people accompanied Joseph Smith when he received the Book of Mormon (none).
Let's consider the possibility that the idea of national revelation wasn't started at the time of Sinai, but at a later point in history, as some Bible critics have claimed. Let's say the Torah was written by Ezra, for example, 1,000 years after the Sinai experience was said to have occurred.
Here's the scene: It's the year 400 BCE. A Jewish leader by the name of Ezra goes down to his basement and writes the Torah, including all the parts about national revelation.
One day, he walks into the synagogue. "Ezra, where have you been?" the people say. "We haven't seen you for a while."
"I've been in my basement, working on some projects..."
"What are you holding there? What is that?"
"It's a Torah."
"What's Torah? We never heard of that before."
"Oh, the Torah is terrific. A best-seller. It's law, history, stories. Take a look, you'll love it."
"Tell us, Ezra," they say, leading to the big question. "This wonderful book - where'd you get it?"
"Where'd I get it?! It says right here in the book: 'A thousand years ago, the entire Jewish nation stood at Mount Sinai and heard G-d speak to them.'"
The people look at Ezra and say, "That's a very strange story. Why haven't we heard of this before?"
"Well, of course, it was a long time ago."
"Well, wouldn't someone have at least mentioned it over the years? Maybe Grandpa or Great-Grandpa? Wouldn't a story as momentous as this have gotten passed down?"
"Well... umm... people forget things, you know."
" G-d spoke to three million people and everybody forgot about it?!"
"Yah, I guess that's what happened."
One problem. The Torah itself clearly states in Deuteronomy, 31:21 - "This song shall testify for them like a witness, because lo yamushu mipicha - it will never be forgotten by the mouths of their descendants."
So they look at Ezra and say, "Come on now. This sounds like a Jackie Mason routine. 'Where'd you get the Torah from?' 'We got it at Sinai a thousand years ago.' 'How come we don't know about it?' 'You forgot about it.' 'What does it say in the Torah?' 'You'll never forget it!'"
At no time in Jewish history would it have been possible to perpetrate a fabrication. It's obvious that the Author put this verse in to preempt such a supposition.
There is a very powerful verse in the Torah (Deuteronomy 4:32-33):
"You might inquire about times long past, from the day that G-d created man on earth, [exploring] one end of heaven to the other. Has there ever been anything like this great thing or has anything like it been heard? Has a people ever heard the voice of G-d speaking from the midst of the fires as you have heard, and survived?"
The Torah goes out on a limb and declares that nobody else will ever even attempt such a claim of national revelation!
How could the author know such a thing?!
how do you arrive at the number 3 million?
Three million is a rough estimation, taking into account the 600,000 males between the ages of 20 and 60 in addition to the other age groups + women and children, (with an average age span of +/- 120 years).
