I am in the process of potty training my 3 year old and I also have a 18mth old, both boys. 3 years does not seem terribly late for a boy, I've heard boys not only take longer, but they start later than girls. I know for my son, he used to play on the potty, going sometimes, not others and than one day it clicked. We are now working on the night time staying dry, but are 100% during the day. I know it is also VERY difficult to put your all into the potty training when you are not only by yourself (I am home alone 99% of the time) and you have another child to care for.
Is there a treat your son REALLY enjoys? For my son, it's yogo's, he gets 5 if he goes BIG pee-pees, 2 if he goes little pee-pee's and 10 if he goes poo-poo. He ONLY gets them when that happens. It is to teh point now that he doesnt' ask for them at all, but in the begining, we went thru a LOT of yogo's ha ha. but they're small, so it's not even half a piece of regular candy.
good luck, it does happen when they're ready, give him some time.