Factory mids/highs will not be able to keep up with 3 12s. Especially factory mids/highs off a factory amp. (THIS IS MY OPINION)....

Considering the speakers are you actually "hear" they are the LAST place you want to skimp...

To keep the bass out of the mids, you could add a capacitor, which would be the easiest and cheapest way. Nothing like a 6db crossover at 100Hz. However, then there is no flexability. Cheap and basic though.

If you use an aftermarket amplifier for the mids/highs, it will probably have at least a high pass filter built in (most do). That will allow you to keep the bass out of the mids.

John