He was probably referring to a 3 way component system, not 3 way coaxials. If you can accomodate the install, 3 ways are fantastic if you get the good ones (Dynaudio, Focal, JL Audio, Eclipse). For rear fill, I'd just stick with either midbasses or nothing at all. You could possibly do a good set of coaxials (good ones with a external crossover) but personally I'd leave them out altogether and invest more money in the rest of the system. A good set of 3 way components will have a dedicated midbass and will be all you need, even high end 2 way components have good midbass. Rear fill shouldn't really be loud enough to improve midbass, the optimum level for rear fill is when you don't notice their presence, they're just barely there.