if you need an expert opinion let me know. depending on how you wire it you can get verious ohm loads but it is better to either series or paralell them all. on my cabinet i had to paralell them to reach the right ohm load. if you series the voice coils on the woofers and bring them all up to 8 ohms then paralell them you will get 2.6 ohms and if you paralell them the whole way you will get .6 ohms its up to you.
Totally depends on your amp. What's the lowest ohmage your amp can handle? You don't have too many options here with 3 DVCs, DVCs are better for a solo sub application. Higher ohms usually means crisper bass, while lower ohms usually means louder bass.