Here's an unpopular suggestion, but it can be effective. Don't buy ANY pull-ups, even for naps. (cringe!! Don't throw things!) Get some plastic pants to wear over the big boy undies, and don't switch back and forth between undies and diapers. The plastic pants help, somewhat, to keep the puddle localized, but won't eliminate puddling and dribbles on the floor or in the bed when accidents happen.
This means more work for you.
Sorry. More laundry, more puddles, etc. but this worked with my two oldest kids.
My third child was just way out of the box...in retrospect maybe we should have waited until he was 4 years old. I'm not sure. Frankly, he would still be on the bottle (at age 6) if we let him, so I'm not sure how long he'd have clung to the diapers. He is emotionally a bit delayed and the whole potty training thing was an interesting time. Ask me some day and we'll hire babysitters and get some coffee and I'll tell the whole ordeal! LOL