how recently? when you say 'nothing is working', how consistent have you been, and for how long?
because when someone says a problem has recently surfaced and they've tried everything and nothing is working, it says to me that too many 'solutions' have been tried for too short a time.
3 year olds cycle rapidly through phases. it's a challenging time to parent them. fortunately it's also insanely fun and rewarding or we'd never make it through.
all i would do with this little perpetual motion machine is stick to my guns. if he stays awake til 4, well, remember that nothing you do can FORCE him to sleep. but he stay in his bed quietly he would. the bedtime routine of bath, story and brief cuddles would remain unchanged (it was my kids' routine from birth until teens), and that's that. when you say 'fuss it out' is he screaming, or just doing the occasional forlorn 'mom!'?
i'd go in periodically and quietly stroke the little restless fellow's forehead, and retuck him in, and then quietly leave again. no talking, no listening to the pleas, no letting him back up. but no getting mad at him either. it's not his fault he's having a jolt in his growing and it's jangling him up.
but don't let the phase rule him (and you.) no staying up all night and sleeping until 11. no keeping you up at night. he can lie in his bed and talk quietly to his stuffed animals, but that's it. keep the routine firm, and the night world quiet, and let him find his way back to a good circadian rhythm.
ETA i'm distressed to see from your SWH that instead of simply creating and keeping a solid, reliable routine for him, your solution is to let him stay up and play and then just give him a pill to fix it. the melatonin should only come into play AFTER you've parented him appropriately.
khairete
S.