If your 3 year old refuses to potty train then you're gonna have to force him to use the toilet.
That might mean a lot of crying, tantrums, accidents you have to clean up etc but you must show your 3 year old who's boss and you have to make the decision that's right for him.
What you have to do is just take the diaper off and not put one back on him no matter how much he wants to wear one.
Remove the diapers from his sight and once you take him out of his last diaper never put one back on him.
Put underwear on him and he will have accidents and you'll be changing underwear and cleaning up urine and possibly poop off the floor but you must insist that he wears the underwear and keep telling him that he will feel much better when he uses the toilet instead of using his underwear.