M.H.
have you tried a more event-like incentive ... Last summer (3rd birthday timeframe) I signed my son up for a toddler soccer camp.... told him all about it and we watched soccer on TV a little bit. He was excited. I explained to him there was one rule. There are no diapers allowed at soccer camp. He had to use the potty and ditch the diapers This was a dramatic improvement within days. My only mistake was that I started this story 2 months before the class started ... it took too long to get to that first class.. so we started to regress .... but once the camp started .... I sent him in underwear ,with success !!! ... and would you believe the 2nd day he asked to use the porta-potty in the parking lot before the class started..... (his dad pointed it out to him the day before.)
After that , he probably had an accident every couple of months...
Now my younger son loves planes... he is 2 1/2 and refuses the potty most of the time. When I told him we can go inside a plane while it is up there in the sky.. he was excited ...... and he couldn't believe I would fit in there too! hahaha
But I told him there are no diapers allowed on the plane. He instantly responded with "ok, I can go pee pee in the potty.". we'll see how that goes.... I think that just proves he could be trained now, if he just decided it is something worth doing.