I completely agree with all the mommies saying that it just takes time. However, if you think they may just need motivation, here are some possibilities that I've had success with (and btw, my son wore pull-ups when he was 3 also, so I really wouldn't worry too much):
REWARD CHARTS: Use stickers to track their progress (let them pick the stickers!) and when they have gone a certain number of nights with no accidents, they get a treat or reward that's already been determined. You can make your own chart...I'd be happy to send you a copy of the one my son uses. It's a Microsoft Publisher document, so you'd have to make sure you have that program. Just message your email to me if you're interested.
NIGHT-NIGHT FAIRY: My mom's personalized version of the tooth fairy...if they go all night without an accident, they'll wake up to find a little surprise under their pillow, left by the Night-Night Fairy. My mom did candy, but if you're worried about dental issues, you could always do something else.
ACHIEVEMENT CHAIN: Cut colorful paper into strips, and for every night they don't have an accident, loop another link onto the chain. We always hung the first one from the ceiling and added onto it. Once the chain touches the floor, you get a reward. Then we'd drape the chain over the door or something fun like that.
Like I said, these are just motivational ideas for kids who might need a little incentive to work harder at the nighttime potty situation. It might work...and it might not. But you're definitely right not to discipline them for this. Reward their achievements and love them when they fail. As long as you do that, you're definitely doing it right! And try not to stress...they'll get the hang of it! Good luck!!