A.P.
I had the same issue just two weeks ago. I was tired of cleaning up accidents and washing several outfits each day. Finally, without any discussion about it, I put him in diapers. I put the potty chairs away and didn't talk about it. About a week later my 3 year old asked if he could use the potty. I let him, praised him and put the diaper back on. By the end of the day he asked to wear underpants. He has been dry ever since, including naps. I still put an overnight diaper on him because he is a very deep sleeper.
It was all about control for him. My ped said little ones can only control two things; eating and potty. Let them make the choice and it will always work out. Kids won't starve themselves and they won't wet their pants their whole life.
Good Luck!