Sounds like you're doing the right things otherwise your two younger ones wouldn't be so good.
I would get your eldest checked out (just in case he has lazy muscles or an immature development of the bowel - it would be awful to find this out later after he'd been hassled etc by you &/or others!).
All that said though, he'll be at school/kindy very soon & the emotional trauma of what other kids would say if he was in nappies or soiling his pants would be horrendous & the effects would be long term as well.
You know your child & so are best placed to figure what would work & to take into account the other two, but I still find bribery to be extremely useful (less so with my son but still worth trying). If there are things he wants(toys/clothes etc), or places he wants to go(movies/park/friends etc), or things he wants to do(play sport/watch certain tv shows) you could use these as bribes/bargaining tools.
Sit him down & explain that he's too old to poo in his pants or in a nappy & therefore from now on he will not be put in a nappy at any time no matter what (but make sure you stick to your decision) & that if he wants to ....... (whatever bribe/bargaining tool you choose).......... Then he has to poo in the toilet. If he poos in his pants, on the floor, in his bed, outside anywhere other than in a toilet then he can't get/do/go.
But this will only work if you explain this all to him & if you are 100% consistent no matter what. It is also important to let him know how good he is about other things & how proud you are of him 7 that you are doing this not to be cruel or nasty but because you don't want him to suffer from teasing etc.