My god No stop it. That is abusive behavoiur , is only going to get the child more stressed, the only way he will do is eith kind word's and a soft hand.
Potty training shouldn't involve punishment.
If you have a 3 year old who is still not potty trained, there are several reasons.
1. the child is simply unable to predict or hold it when he has to go
2. he's upset and he's "sticking it to you"
In neither case should you punish him. If you suspect it's the second case, that he's upset and it's showing in refusing to use the potty, you can have natural consequences like having the child clean up the mess.
I have to say that this could be viewed abusive behavior-- as a social worker in my state-- this could be viewed as abuse to the child by state authorities