Where are you flying to, the space station? Three months sounds like a bit much, unless you're unemployed an picking change off the ground. Contact the airline. Most of them charge a fee to change the terms of the ticket.

You gotta do both cause that person sees you as their best friend and they obviously want you to be there if you're the best man.

you simply have to do both, i flew half way around the world to be best man at a wedding then flew back to continue my holiday... if you seriously have to think about not going to the wedding because you're not there and your elsewhere then i don't think you deserve to be best man

Go see family. Weddings are dull and being a best man is a thankless task. Save your money.

Sounds like you don't have an option you'll have to fly back early ! Sorry to hear man but you'll be a great best man plus bachelor party will be fun