mixing parallel and series wiring, 3 subs

Here are the general guidelines, with few exeptions:

• all subs shall be identical models; same make, size, and voice coil configuration
• all subs shall be wired identically (or such that each sub arrives to the same impedance as all other subs, as in the case where a set of coils is polarity-reversed because the driver is inverted)

If you are stuck with an impedance problem because of your choice of subs and the number of them, you will need to make changes in your system.  Different amp, multiple amps, different subs, different number of subs...

What subs?  What amp?

You need to pay extra attention to the layout of the wiring if you mix series parallel.

For example, say you have 3 4 ohm subs and you are trying to get to a safe load to wire to a 2 ohm stable amp.

If you wire two subs in series you get an 8 ohm load and if you parallel that with a 4 ohm sub you'll get 2.6 ohms.  Your load is fine, but your power distribution will not be equal at all to the subs.  The 4 ohm sub will get significanly more power then the rest of the subs.

As a rule of thumb here NEEDS to be an even number of coils involved to wire series/parallel.  You need to make sure you have the same number of coils in each "leg" that is going to be paralleled together to make sure all subs recieve the same amount of power.