If you're powering the subs with subwoofer plate amps, which already come with low-pass crossovers, all you need is two high-pass crossovers. There are a variety of things that do this, but one of the cheapest and most configurable is the Behringer Feedback Destroyer Pro. Don't let the stupid name fool you; it's a parametric equalizer designed for pro-audio applications, and, as such, is not as overpriced as a lot of audiophile gear. There have been three revisions, but all are fine for bass duty, and you can find them used on eBay frequently for as little as $65.
If you're not using subwoofer plate amps, you would need two of these in your proposed setup. I would suggest however going with plate amps so you can get variable phase control, otherwise the integration of this scheme is going to be quite a nightmare.
The great advantage of the BFD is that, being a fully featured parametric EQ, you can use it to help control the room response nodes as well as just do the crossover.
That said, I question the wisdom of your three subwoofer solution. It's going to be very hard to get cleanly integrated. You have the two larger drivers handling just a little over an octave, and the smallest driver not even handling a whole octave. It's complete overkill. I know you're looking for low distortion, but the tradeoff will likely be poor integration. You would be better off going with just two drivers - a 12 inch and an 8 inch - with a single crossover point in between. Drivers can handle at least two octaves. If you're into overkill and extreme low distortion, build both enclosures sealed and add a Linkwitz transform to the 12 inch driver (you may need to bump that up to a 15 inch driver under this scheme to get the SPLs you want). Incidentally, the BFD can simulate a Linkwitz transform too.
