Hi! I'm sorry you're going through this. My son is now 7, but when he was three we went through a similar situation. Finally I watched a Dr. Phil show that gave some ideas, we tried them and they worked. My Husband and I planned to stay home all weekend to work on this - we called it the Naked weekend.
Starting early Saturday morning we took all of his clothes off and put the potty chair on some towels in the middle of the living room. We read some fun books about potty and made the whole weekend about this. We watched him very carefully and didn't let him be alone or behind anything that we couldn't see him. He wasn't happy at first - a few times he would start to go and we would rush him to the seat. There were a few messes on the carpet which we just did a basic cleaning on and called the carpet cleaning service on Monday to do a good job.
When he did get some in the potty we made a big deal about it - clapping, gaving him special treats to eat and I had put a few balloons in a closet the day before that he didn’t know about and brought them out! We made it like a party.
That night we put him in bed with regular underpants and a waterproof sheet. He didn’t end up going through the night, but if he would have our plan was not to change him until morning. I know that sounds really bad and it would have broken my heart, but that was the plan.
By the second day it was a little easier to get him to go in the potty, we still made a huge deal about it and my sister brought over a special toy for him as a congratulation gift for becoming such a big boy… By Sunday night the potty chair went into the bathroom (make sure you have one in every bathroom so there’s never a delay). We only had one other accident the next week and he’s been potty trained ever since.
Some kids you need to add a few days to this plan, but in most cases if the parents stay strong – it ends up working. It’s NOT Easy, but it worked for us. Good Luck!