The biggest obstacle to making such a do-it-yourself antenna combiner set-up is that DirecTV splits the Ka band, frequency-shifting half of it to frequencies 700 MHz below the L-band, and the other half to 700 MHz above the L-band section most commonly used by DBS satellite.
You could probably combine the signals from three conventional SL3 LNBs by covering over the two undesired feedhorn openings in each. The only way I see that running into a problem is if there is AGC circuitry in the SLM3 LNBs, but I don't think there is.
The benefit of doing that is that he could then use larger, higher gain dishes, but unless he is planning on using really big dishes, he should just buy the Alaska/Hawaii, 1.2 meter dish. It isn't that expensive.