You will end up pushing him away if you don't relax
He's almost an (new) adult at 18
You can have reasonable house rules- don't make them too strict or he might prefer his Dad's house - about not waking household up - but you don't get to give him a bedtime 
now. He'll have I learn the hard way that lots late nights=tiredness /for lectures/work . You can't parent that into him if he's not listening nor has need for it at the moment
He's just chilling with his friends. Lockdown has made their sleep schedules topsy turvy- he's probably finding a quiet outside space for them to chat ,have a couple beers , etc. He's meeting up with girls? 17-18 yo boys do that 😊
3 am is late but he's enjoying his freedom to come and go. He's off to uni soon anyway. Let him.
Just talk about making 'safe devisions' 'good decisions' , that it's be good to see the sunlight for some of the days 😂, can he call you if has any trouble or needs you, say it's been hard for you to learn to let go and not worry but could he come in quietly and just pop his head round your bedroom door today hi when he's in. You'll probably find after a while it no longer keeps you awake and you sleep better.
Can you not remember how much fun it was to be his age and free to come and go without curfew? How hanging out late at night with friends in the early hours was slightly magical- it's quieter then.. less people around.
My son is almost 18, uni in Sept. He does this (mostly 1am sometimes later) , he tells me what a great night they had, sometimes even shows me photos, they're laughing in them, telling in jokes, occasional beers, nothing that would worry me. I let him/his friends/ have bbq get togethers every couple weeks til late in my garden (within reason and respectful to neighbours) (I text him of they are too loud after 11.30pm so not to embarrass him) (except if they stay too noisy I do come down in my pjs and throw a few dance moves (tiktok) and say all us old folk will join in if you insist on keep us awake! It works..! 🤣🤣)