Hi there!
I can truly empathize with this struggle, being a father of 2 toddlers.
I normally do advise parents that inculcating the habit of brushing starts as early as when they are a newborn. Introducing them a towel or a gauze to clean the bony ridges daily even before the teeth erupts. This is to have them understand that it is a routine for us to clean our oral cavity.
Now that your son is 3 and past that stage. It will involve much reasoning. Find out why he hates brushing. Also try to make teeth brushing fun. Try to get toothbrushes that attracts him like his favourite cartoon characters or toothbrushes with fancy gadges/lights. This will definitely pique the kids' interest.
Avoid making the dentist the boogeyman as that would make them fearful of visiting the dentist when it is necessary.
Hope this helps!
Cheers,
Kenneth
