If it were my 3yo, I would say Thomas trains. He collects the Trackmaster set (plastic battery operated). You can get the from walmart or target for under $15.
I found something at Target made by Alex Toys called Rub A Dub in the Tub or something like that. Anyhow, it's got all sorts of foam train pieces that stick on the tub and the little boys we have given them to love them. I think it is about $12. They have others with flowers, fashion dress-up, etc. They come in a little bag that sticks to things....They also have them online, at speciality stores, etc.
Re: gift idea for 3 yo boy?
My DS is 2 1/2 and he loves anything Thomas! Books, trains, etc....
He also likes DVD's of Elmo, Thomas, Clifford. He also loves books and anything with trucks/cars.