Ah, the joy of 3 year olds. I love two year olds (they are just trying to figure stuff out), tolerate 3 year olds (still new to everything, but beginning to test), hate 4 year olds (they like to throw everything at ya), and get back to liking them as a 5 year old. But I must be insane as I'm looking at buying a 3 year old here shortly.
Anyways, best of luck to you. The main advice I would give you is to make sure you are firm and consistent with these youngsters. they really need consistency, where you always ask for things the same way and respond to their mistakes the same way. But they can be so much fun, and so rewarding to work with. If this one is at your RI place, don't be afraid to ask for help.
As far as the lunging goes, just take baby steps. Work on mastering the walk, stop, and reverse first. Work on trotting this one by your side. Stay right next to him, and when you ask for the trot, give a verbal cue, move forward, and tap him with the lunge whip or lead line on the behind. But if he goes forward, make sure you stay with him. It will take a while, but he'll get the idea there, and hopefully associating it with a verbal cue will help transition it over to the lunge line.