Well, I'll assume best case cenario...all in seperate enclosures, and all 4 ohms. If you want to give your 15" enough power to get any reasonable performance out of it, it will need 500 watts or more, but your 6000's can only handle 250-300. About the only option using one amp is to run your subs paralell for a 1.3 ohm load. Let's say you use a 1000 watt amp. All 3 subs will see 333 watts, too much for the 12's and too little for the 15. I'm sure it would be ok, but I just don't think you'll be getting the best from either the 12's or the 15. If it were me, I'd sell the 15 and use that $$ toward one sweet amp for just the 12's. Or sell the 12's, get another 15 and an amp for those.