K.A.
I think swimming lessons are the most important "activity" a 3 year old should be doing. Anything individual or done in a group, that is fun, loose, and not too regimented would be good. Maybe something creative like a music or dance class. I would not recommend soccer or a team type thing... too much inactivity, stagnant time, too much instruction, etc. for a 3 year old's attention span.