Ok, so my son turns 3 next month.
we let Logan pick out his own underwear :) He potty trained right around 3 years old... We really tried everything... having him try to go on intervals... candy when he went potty... a sticker reward sheet (no accidents for 5 days got him a prize like going to Chucky Cheese or a new toy)... but really in the end, it just clicked for him and he just became potty trained. LOL
I know a lot of people try different things.. it'll be figuring out whats best for you and your son :) Good luck! Keeps us posted :)