We've had the same with DS (2.5). DH had more success than me but we've tried loads of different things you've probably been through:
take them shopping to choose their own toothbrush (boots had some with a head that clips over the brush when finished - pink cat or yellow elephant I think - DS had both at one point!) and make it clear that you're buying one brush for them and the other brush for yourself to clean their teeth with (make sure the toothbrush you are going to use is really boring looking so they don't want to swipe that one too!). (Can do the same with toothpaste, luckily DS likes the mint so we've never had to go down the strawberry flavoured route as I know we'd have struggled to get him back to mint, but we did get an igglepiggle toothpaste in boots, which has helped).
most of the kiddies programmes have one episode on cleaning teeth at some stage - we've got them recorded - balamory do a song about cleaning teeth, teletubbies have one, Charlie and Lola, so these are the episodes we watch more often... also, DS doesn't like characters like Spud in Bob the Builder, the goblins in Noddy, Swiper in Dora, even the goat in Postman Pat who bit off a teddy's ear - so DH started telling him if he didn't clean his teeth Spud would come and live in the dirt in his mouth! - somehow that one worked.
Also, I have one of his toy phones in the bathroom and say I'll have to ring the tooth doctor if he doesn't clean them properly. (Don't say dentist as don't want to put him off that one!)
Another good one, is get a mirror at their height so that they can look at themselves cleaning their teeth.
A friend said she used to tell her DS that the monsters would be coming to pinch his teeth if he didn't clean them.
Also second the suggestion of letting them clean your teeth, I would let hold the brush so he could feel how I was brushing then he would have a go (ahh, my poor gums!), and I often try to clean my teeth at the same time as him even if I've already done them.