My cousin, around that age (as well as younger and older) was obsessed with - wait for it - vacuum cleaners. Like, my aunt tied a string onto an old toy broom or something to try and make him a "toy" vacuum cleaner.

So not much help in suggesting things for the bull obsession (what does he think of Buffalo/Bison? Because they pop up a lot around here - I think they're the semi-official animal of the province), but I wanted to let you know that there are weirder things for a three year old to be obsessed with...