my baby is nearly 3months old,he likes to put his fingers inside mouth to suck is it because his growing tooth or something else?I'm wondering...

At that age, babies are discovering their fingers, feet and pretty much how their whole body moves and functions. He is realising how his hands work and how useful they are. Also, he is probably discovering how soothing it is to suck on his fingers :) It's very normal behavior and up to 1/3 out of babies suck on their fingers.