MINE - Monster

You can start telling him that some things are just his and he doesn't need to share, like certain toys, or a single cookie, but something like a box of cereal or a jug of milk is a different story - this is food that you have purchased and is for the benefit of the whole family, not just for him. If he starts flipping out over "sharing" something like that, remove him from the room and put him in "time out", telling him he will stay there until he calms down and decides to be act nicer.

sure, some stuff IS just his. some stuff, little sibling is too young for. make a big deal about those being HIS only. then reinforce good behavior for being a "good sharer" - and yes, if he's throwing a tantrum about "sharing" his cereal for breakfast, then i would send him to his room until he can share nicely. when he gets hungry, he'll "feel" like sharing! play up taking turns, compliment him on being such a good big brother when he shares...

All of the above. So many times you just have to do what works in the moment. Taking turns usually works best with my boys (and my two neices, who are over 2 days a week). If we have two of something, great. But we don't have two of everything, so we have to split it or take turns.

As far as a meal or box of cereal, you could try plating their food and giving each a plate. I do that at restaurants. I often do not get a kids meal but have them share and regular meal (usually more food and cheaper). I also pour their cereal and place it in front of them.

our kids aren't too bad about that. but... when i get drinks or anything the 3 year old will ask who its for, and i will explain it is for everyone, not just him. and with toys, he wants them all the time, so i will tell them they have to share. he will come to me and tell me he wants it, and asks if he has to wait until his brother is done. my daughter doesn't always like to share. when she refuses, when any of the kids refuse they get the toy taken away and have to stand in the corner. that usually takes care of it pretty quickly.

My children are 2.5 years apart.
My oldest had a case of the mines as well. I told him that every time he was selfish, he would be the last one to take a turn and if he threw a fit, he'd have a time out and he would have to wait longer for his turn.
It worked.
I think I only had to enforce that a few times.
Eventually, he would say, "LLB can have it first." Sometimes if he said that, I'd reward him and say, "LLB had it first last time, this time it's your turn to be first." He got the picture!!
YMMV
LBC