i have to say that i strongly object to the attitude of the person who wrote "time to get real." i think it's glib and excessive and not very nice.
MANY three year old boys are not capable of using the potty. while i have heard of the occasional PTd 3 yr old boy, the overwhelming responses of moms I know, who have gone through this, is that boys consistently learn the potty much later than girls, often as late as 4 yrs old. my son is 3 yr 5 mos, and is not only resistive to the potty but also not aware of his body signals for the need to potty in advance. he is often surprised to find himself in a dirty diaper if he is concentrating on playing or reading his books.
you should read Dr. Spock on potty training, and look at such websites as Kellymom.com and Askmoxie.com, and also read Dr. Sears. you will find a much more reassuring, compassionate approach to this milestone.
your son may very well want to wear underwear and that's great. encourage him as much as you can, offer him many opportunities to use the potty, and relax. as my mother would have said, "You never saw a bride in diapers." Surely, neither a groom.
good luck!
J.