Have you tried an electric toothbrush? My 6 year old loves his (he got Spiderman), but my 3 year old is afraid of his. Still, it could be worth a try.
I am just recently getting the 3 year old's teeth brushed without a fight. I wish I had advise for you, but I just tried to be consistent. We brushed every night, if he gave me a hard time I just did as much as I could without him getting too upset (judgement call) and let him go.
I had to use different tricks with him that with big brother when he was that age. With big brother I was able to brush his front teeth and gums be getting him to say "cheese." With this guy, cheese gets me a split second, so I have to tell him to say "ah" quite often, but it gets the job done.
Patience, consistency and perseverance. I guess we could all use more of that :-)