Preschool and Potty Training

I've been debating for a few weeks whether or not to send Evan to a three-year old preschool program in Nevada 2 days a week. I think it would be so great for him to get out of the house and interact with other kids his age. But of course he needs to be potty trained to go. We haven't really started potty training yet. We usually ask him at every diaper change if he needs to go potty, sometimes he says Yes and he goes, sometimes he says Yes and he doesn't really need to go, and not very often he says No. Thankfully a majority of the time if we ask him he says Yes and he actually goes. He usually has one or two days a week that he stays almost completely dry and only poops in his diaper. So I'm hoping to be able to get him potty trained so that he can go to preschool this fall.