Said on the basis of what? To me, as a non-Christian, the concept of the Trinity is both perfectly simple (three aspects of the same thing) and perfectly monotheistic. I was baffled once as to why muslims cannot understand it that way until realising it was just far more convenient for them not to bother to try.
If Christians believe they are worshipping one God then that is precisely what they are doing. What muslims, or anybody else, may think they are doing is totally irrelevant. It is as arrogant of muslims to lecture Christians on Christianity as it would be for Christians to lecture muslims on Islam.
And also for Budhists to lecture both muslims and christians. Don't you see that you are just doing the same? Anyway as far as you perfectly understanding the trinity, then this shows that you too have no clue about monotheism.