impedence with self made crossover

If I used multiple drivers, like in a 3-way design, using a woofer and two phenolic compression drivers for mid/tweet, and hooked them to a crossover that I made, what impedence would I have if each driver is 8-ohms? I know if you average it out, like if they were all linked directly, you would have about 2 2/3 ohms... does hooking them up to any crossover make that impedence 8-ohm always? How is it determined? What must I add to make it 8-ohm? What about 4-ohm?
The crossover will present a low impedance for the passband for which it is designed, and a high impedance for the out-of-passband frequencies. It does the summing by itself.

Not to be a smart-aleck, but pick up a copy of the ARRL Handbook in virtually any library in the US and they go through the physics of why this works in terms which we can all understand.