Most start at the kindergarten age. Some kids start too early and end up not liking the sport, others are ready to go.
Keep it fun for your 3 year old. Hit him/her balls, pitch wiffle balls to the young ball player and have them throw and try to hit targets. That in itself can be a lot of parent/child fun and learning too!
My boys (twins) are now 6 and they've been playing since they were 4. You have to go to different parks to play at the different ages. I coached and the boys played at Matthews Park (next to the YMCA on Micheal Street). We then decided to move the boys to Westside when they became old enough. Westside has the best/most competitive program in Mobile. I wish you luck... remember that T-ball isn't about winning but it is about learning and progressing.
Hey it might be to late this season, but boykin park has wee ball for 3 year olds. My daughter played in the spring and loved it. They also have fall ball coming up and they might be doing wee ball in the fall too, not sure though.