When using the keg kit do you primary in a keg or only secondary? How do you get the 3” of true out?
So I ferment 10 gallon batches in a 15.5 gal Sanke. I generally calculate my recipes for a little overshoot on volume so I am assured to get 2 completely full cornies, so my recipes are generally for 10.5-11 gal final volume. I'd rather leave a little leftover than miss a full keg, especially since I don't brew as often as I'd like to. Time>money.
I use it as a primary. I don't secondary. Since I serve out of kegs, the kegs will continue to clear in the fridge.
As for the 3" of trub [I assume you meant trub rather than true], basically I'm racking off the top like you would with a standard racking cane. The kit allows you to move the racking cane up and down so you can monitor the beer coming through the tube into the keg and you know whether you're pulling clear beer or trub. I can fill both corny kegs before I start getting down to the trub.
After transfer, I just pour out what's leftover in the Sanke. If I want to re-use yeast, I keep a sanitized vessel and pour some slurry into that while discarding the rest.
FYI the other advantage is that I have my blowoff tube rigged up to a MFL fitting, so I can hook up a CO2 tank directly to the tube. So I transfer under CO2 pressure rather than needing a siphon. This way I never have to lift the fermenting keg up high; I can just leave it in my fermentation fridge. As a result, I don't disturb the trub, resulting in clearer wort during transfer.