Potty training comes and goes. No 3 year old doesn't have any accidents now and again, so don't stress yourself too much. Pre-schools know they have to ask them to go potty. Often they will take the children to go potty about 5 times a day on a schedule. I always packed extra clothes and plastic bags in his backpack just in case.
I would try giving him lots of fluids during the day, ice pops, and continue to take him to the potty every 30 minutes. You can use stickers for rewards. A friend of mine said she used M&M's. Her son loved candy, so he would keep sitting on the potty just to get an M&M. She would reward him 1 candy for sitting down. After he started sitting down like 30 times a day (lol), she started only rewarding him when he did something.
I personally reward my son for sitting down, pee-ing, pooping, pulling up his underwear on his own, and pulling up his pants/shorts on his own. My 3 year old does pretty much everything on his own and is very adament about doing so. He was fully potty trained and now we are back to pull-ups. He comes and goes.
My 4 year old is finally fully potty trained. He still has accidents though. He gets upset or really into whatever he's doing and holds it too long. I still send extra clothes to pre-school and daycare for him just in case. For the most part he does very well.
When kids are sick, they tend to have more accidents. After my boys had tonsels out, they both reverted back to pull-ups. When they had the flu, they reverted. When they had "the runs", they both reverted. So... it comes and goes. Just stay consistent. They don't like to sit in wet and dirties, so sometimes I ask my boys if they pee'd or pooped their pants. Sometimes they even asked to be changed.