What do you mean by 'keeping your Medicare' here? I am assuming you are referring to Medicare in the States, but what happens if you fall ill in Thailand and are unable to get home?
I hate Phuket but lived in Nai Harn not Patong. Phuket is expensive, infested witrh Russians and the natural habitat other than the beaches has largely been detroyed for tourism. I suspect you have a reason for living there and after 27 years you are probably nicely set up there. For a relative newcomer looking for somewhere to live out the last years of life, it's not a place I would advise him to move to. In fact out of the three places mentioned, I'd choose Bangkok, but I am sure he could find somewhere much more leisurely and affordable elsewhere in the country. Phuket is also the one place in Thailand in which your own transport is an absolute necessity. At 76, you don't want to be negotiating for a fair price with the local transport mafia.