This is the reason why JL Audio came out with the W6 series of subwoofers. Each driver has two 6 ohm voice coils. In order to achieve an impedance of 4 ohms with threee drivers, one would wire it like so: Wire the two voice coils of each driver in series. This will result in an impedance of 12 ohms per driver. Then take these three 12 ohm loads and wire all of them in parallel. This will result in 4 ohms.
The term driver that I use describes the actual speaker, you know, made up of just the frame, cone, surround, voice coil, magnet and spider.
A set of three 10W6's would usually fit nicely behind the rear seat of most cars.