Someone correct me if I'm off on anything
The sIII is the Trinity version of a Marshall 2 channel 18 watt TMB. a little less gain than some of the other TMB's out there.
The TMB (I believe) is somewhat higher gain
The Plexi has a more Plexi'ish preamp section.
The differences between these three are minor and in the pre-amp (again, I believe)
The SIIIv6 has both el84 and 6v6 power tubes that can be chosen between by a switch on the back of the chassis (either the 2xel84 or the 2x 6v6, not both sets at the same time).
The TC-15 is a Trinity take on something like a Matchless 1/2 power DC-30 / Vox 15. Has an ef86 channel and a ecc83 channel.
RS clone refers to the custom wound transformers Trinity has that are based on the original Marshall 18 watt RadioSpares transformers.
I've built an sIII for my nephew and an SIIIv6 for myself.
Stephen provides GREAT documentation and parts selection
Ho[e this helps, anyone please correct any errors!!