A., I feel your desperation and frustration, however here are some things to take into consideration....
Is there a reason he needs to be potty trained now? If a child is ready to be potty trained, he/she would seek the toilet and know that he/she does not like being wet. Reading your post, it is not that he is ready but you "want him out of diapers as soon as possible." Remember that Potty Training is as much a psychological thing as teaching a lefty to write with the right hand. It is one of those things that you have to let them just say, I've had it with being wet, I NEED THAT TOILET. Where my eldest, who is now 11, was potty trained at age 4.5 yrs, the twins, who are now 10, were just a few months shy of 4 yrs when they were potty trained. The 2 younger ones, who are now 8 and 5, were about 3.5 yrs when they were potty trained.
It is very heartbreaking to hear "I'm sick of changing him all the time especially when i know he can use the toilet, but just doesn't." They don't use the toilet for many reasons. It could be they are hanging on to the attention, toddler years, not ready to be "grown up", many number of things. He will grow up feeling inadequate if he sees your disappointment, let me tell you that DOES carry on to the rest of their lives. For his sake and yours, allow him the space to do this at his pace and not yours. Our children will never be at our pace. In retrospect, given my kids ages now, I am wishing they were still in diapers...lol. That age will never come back again, take your blessings and run with it... you have 3 of them who want and need you, that day when they say "Bye MOM, see you in a few days" is just around the corner.