Tom,
I believe you are calling the track power from the booster to the track as the track bus.It's simple. Daisy chain the boosters using the COMMAND BUS. Each level will have one set of power leads from the booster to the track. (Although I recommend breaking out each level into power districts using a couple of zone breakers)
Now why double gap each level at the track? If you don't the current will add together. So 5 amps from one level will add to the 5 amps on the next level, and 5 amps on the 3 level. That's 15 amps. If your train shorts a gap, that's a potential for a fried decoder.This plus slight differences in voltage between the boosters can burn them out early. When you double gap your track between levels, be sure to do a voltage test between Level 1 and Level 2 rail voltage. It should be < .1 V. If it isn't you need to either run more feeders to reduce voltage drop, or adjust the voltage using the trim at the booster. The investment in opto isolated boosters is also worth it.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!