My son is also 3.5 and acts JUST LIKE THAT. He is strong willed, smart and determined. If he decides he is going to do something, he will. Setting expectations, giving consequences, being consistent - giving positive statements (stay in the cart) and all kinds or reminder discussions (When we go to X's house, is it ok play in the toilet? Is it ok to share a toy? etc) all help, but my son still is SOOO HARD. If you enforce a rule 39 times out of 40, he will keep pushing again for the one time exception.
I do NOT think there is anything medically wrong with him - I think its just his personality.
My daughter, now almost two, is NOTHING LIKE HIM. If you tell her one time "No" she is like, "Oh, ok." even if you had told her yes another time. It sooo easy to take her places - fun and relaxing! With my son, I have to ALWAYS be vigilant.
There is a great book - Maybe Parenting the Strong Willed Child that gives good tips and lets you see that kids personalities are just different - there is nothing "wrong" with them, but you do need to take different approaches when disciplining.
See my recent post about my son during Thanksgiving and see if it sounds familiar: http://www.mamapedia.com/questions/12873515707098398721
You are not alone mama! Next time you are frantically looking around a grocery store for your little escape artist, hoping no one will report you to CPS for losing sight of your child AGAIN, know there is a mom in Dallas wishing for some duct tape...