My 3 years old boy started to pee in his pants again. what could cause this?

Many causes: Some of them:stress, uti, diabetes...

Not abnormal: We see this all the time. Often with really intelligent 2 - 3 year-olds. Some kids realize that their eliminations are 1 thing they can control in their lives. Others are just too engrossed in their play. Others know that they can still "hold it" after 1st sensation of bladder fullness & then forget. But do have urine checked for uti. Consult dr if develops neurologic symptoms such as tripping.