If ur going to sleep at 3:30 then you should wait for Fajr. Thats what I used to do earlier this summer and then I started sleeping at a normal time like 11pm so I was able to have a 5-6 hour sleep before Fajr. But now I'm starting to fall back into that bad habit again of sleeping very late so I might just start waiting for Farj time to come at 4:15am. But it is a very bad habit and I hate doing it. But I know if I go to sleep at like 3am, theres NO way im getting up for Fajr.