My son is pouring things on the baby's head...

It is a form of sibling rivalry. The 3 year old is probably looking for attention from you. That's not to say that you aren't giving him any, but he is testing you here. My 8 year old and my 4 year old are constantly fighting with each other. Once in a while they sit together and play a game, read a book, etc. but most of the time they are in a small war against each other. It's just part of being siblings. You may just want to not leave them alone in a situation where you can't see what's going on for too long.

um... it's just his curiousity... not jealousy... perhaps he got his idea when when he watch funny tv programme and just try to be funny. well children do what they see without harm intention... or did you laugh when you see the first accident before telling him an advice? if you laugh then he think it was okay even you advice him otherwise.

I don't think its anything rivalry , your three year old is also too young. and may be curious about this new fellow in his life and may be he has seen you grooming the baby with powder and lotions so he is trying to do so too. He obviously is too little to understand that baking powder is not a powder. he has seen you do so and is just copying the same thing. May be its time you start asking him to help with the baby when you bathe him or groom him. just pat some powder on his back or just hand you the lotions. He will definately feel proud to be trusted with such things and then it wont be only you lookign after tha baby but both you and him lookign after the baby. It will surely help them to bond. I did so with my son when my daughter was born and he adores her like anything and acts like the Big protecting brother.

My son did something similar with baby powder when he was 4, but he did it to HIMSELF. That was a mess. At least it was something you can easily clean up and not really harmful to the baby.