It might be worth it to experiment with a lower dose of Melatonin -- 3mg is still way above the .3-1mg dose usually seen as 'most effective'.
(See https://slatestarcodex.com/2018/07/10/melatonin-much-more-th...)
I switched to 300mcg or 450mcg and it just keeps working, no tolerance whatsoever (actually, after 4-5 days of getting into a sleeping routine with melatonin, I find I no longer need it and I can just fall asleep on my own).
300mcg means I don't notice the melatonin working, I just fall asleep.
450mcg of melatonin -- I actually feel it kicking in like I'm a child wanting to stay and watch the movie, but I just can't stay awake.