A three way speaker may well have bass down to a certain level but not the bass extention that a good sub can bring to the party.
As for blending a sub with mains, it takes a little time and patience. But, I've done this with 2 way, not three way, speakers. Somehow, I seem to love 6 1/2" drivers. They go low enough to play standalone and still satisfy and the sub simply takes over where the mains fall off.