12 inch subs? 2 or 3?

2 ported

For me, two ported every time. And no, you don't need a multi channel amp to run multiple subs. You're better of with a monoblock, especially when your subs share box volume.

i'd do 2 ported.

How much are you buying the 12w3's for? The 12w3v3-4 was my first sub, and cost me $250. Then I bought the VVME RD-12 for $55 shipped and it blew the 12w3 out of the water...in the box designed for the 12w3...

You can likely get a single 12" or 15" that would take up the same space as 2 or 3 of those 12"s (which need 1.75 ported each) and be louder than 2 of them. Even if you are on stock electrical, you could get an amp that would only draw about what your alternator puts out, and run a sub that is a bit more power than that. (I'm running a 1400w rms dc lvl 4 m2 12" with a saz1500d v1 on stock electrical in a 2010 HHR, which has a 4 cylinder motor and I belive a 130a alternator and it rarely goes under 14v)

Whatever you do, please don't buy JL or kicker. Too over priced, and not built as tough as other brands like sundown, DC, DD, IA, FI, AB, etc...