1) It is possible and people who completed that don't even want to hear about doing it again, even as service
2) $800 for that request is spit in face and it's absolutely disrespectful for complexity of that work. That's price for low ranged no prayer cape, not 3 combat cape.
3) 3 combat capes usually aren't done in first attempt even if it goes all smooth. There's chance for staying 3 combat, not any guarantee. If I remember correctly there's 1/16 chance of staying 3 combat.
So yup, if you want to get it done for $800 then you'll have to learn how to do it yourself, If you'll be successful LMK, I'll get you some requests for 5 grand each, I'll happily mediate transaction