I don’t want to ruin their relationship but there’s a part of me that says “get him for yourself!”

If you don't know him too well and can avoid him then I suggest trying that, sometimes it helps to get to know the couple better so you come to an understanding that their relationship is happy and so you're happier for them. Sometimes though, it helps to distance yourself so you don't risk any chances of becoming any fonder of him. I would avoid ANYTHING that might make you take a chance and be like "Well let's see if I can get him", cause you can't. He's with somebody and if he's happy then you should help him stay happy.

Oh he’s one of my friends

My rational part of me is saying that I shouldn’t mess with them, and I won’t, I just need to get over him so that I can continue on normally

i mean the conventional old fashion way of doing that is like, focus on something negative or uncomfortable or unpleasant like him eating lunch meats loudly or something but knowing old fashion things there is probably like bad side effects of doing things that way because humans are stupid

the easiest way to get over someone is to have someone new to be interested in

You have to ignore the "steal him away" part of your brain because statistically if you do get him it is only a matter of time before the same thing happens to you. Think about the things you like him for, and then remind yourself that those qualities are shared by literally millions of people; then take a little time and effort that you would put into him and put it into yourself. Pick up a project you've been putting off, fix something that has been bugging you (it was smoking for me last time I was where you are), or make a (non-drastic) change in your life for the better. People respond to positivity, so love yourself and life and keep putting yourself out into the world. I always remember the quote from Mr. White in times like these "You're gonna be OK! Say the god damn words!" Sorry for the lengthy response, I have been through this a time or ten myself. Good luck!

I too have had many crushes on a number of my friends throughout my high school days. Never a once did anything about it. Just let myself sorta halfway enjoy the weird crush brain happiness

@bethorien gets it