I have tried everything I can think of to get my boy to brush his teeth,but he only will do it when he's in the mood.Does anyone have suggestions about how to get a 3 year old to enjoy brushing his teeth?:D

I'm afraid it's something he has to learn HAS to be done. At 3 the parent should be brushing his teeth. Boys need parental help until they are 11 or 12 since their dexterity is slower to develop.
I have a 3 yr old grandson who doesn't like his teeth brushed. When he is here, Papa holds him and his hands while I brush... he cries (grandson, not Papa) but at least his mouth is open which makes easy brushing!
Eventually he will learn that this is something that MUST be done.