My 3 month old baby seems to always be laughing. What is she laughing about? Does she already have a sense of humor?

Sounds like a cheerful and happy baby! :) Babies usually could start finding things funny from around 14 months on. Every silly act that you do which elicit a giggle or a laugh is him/her learning and developing his/her sense of humor. You child may even try to make you laugh as she develops. Sounds like you and her will be having a fun time!! Enjoy!! :)