I think i'd try do everything I havent been able to do, with a nice big loan from the bank that I would of agreed to pay a few years later lol
If I was told that I have three months to live, I would call my doctor a big fat liar, call the police to have him arrested, go back home to have dinner, and start the next day building the first fully operational mecha, no matter what it takes. After threee months have passed, I will have finished building my impressive project and have it on display to the greatest scientists in the world to stare at it with awe in public, as I happily await my death in engineering and technological glory, only to find out that the doctor really was lying when I woke up in the next day.
Loved that movie. I like that. I would appreciate every little thing that I have in my life. Taste all the foods that I've always wanted to. Visit the places that I've wanted to see. Look for things that make me smile in every moment. Tell the people that I care about, that I love them. Hmmm... actually, there's no reason why I can't do any of that now.