If you have any concern that there has been some form of abuse please investigate.
However could he be doing this just out of curiosity and now since he knows that this is getting him attention, although negative he is doing it more frequently?
My son, who is 4 and is very bright and naughty, started peeing in the tub, in empty bottle caps, in coffee mugs, flower pots etc a few months back. I did not think about abuse since he is at home and with me, but for some reason I did not even react (I guess I am too tired to react to most things). One day he had to go pee pee while we were in the car and there was no way we could make it to the rest room without an accident, I asked him to pee in an empty plastic water bottle in the car, which he did, and since then he has not peed in any odd place at all. I think there was no fun in peeing in odd places if mom was not reacting to it.
If you think there has been opportunity for abuse from anyone you could check it out, have you asked him why he does it ,he is supervised at 3years old does he do it infront of you,?to be able to do this as you describe is he alone ,how about day care or pre school if he goes to one do they have the same problem. Something could be upsetting him,especially as it happens every 4-5 days, what happens prior to his behavior?