Can you set up a layout to run conventional, TMCC/Legacy, and DCC locomotives in 3 rail? Yes.
Can you run them all on the same track at the same time, powered via the track? No.
Some very basic ways of describing the different systems:
Conventional locomotives run just by changing the voltage to the track. Many older locomotives have AC motors in them.
TMCC/Legacy require a constant 18V AC to the track. The track is one half of the command transmitter antenna, the other half is your household wiring. Locomotives have onboard antennas to receive that signal. TMCC locomotives can have AC or DC motors. Legacy are all DC motors. AFAIK there is only one 3 rail O Legacy locomotive with DCC on board, and is the new hybrid brass 4-4-0s.
DCC requires a constant voltage to the track, the command signal is sent along the track by modulating the voltage wave. The locomotive can read that modulation. They have DC motors.
You can not have conventional and TMCC locomotives on a track with a DCC signal.
You can run TMCC and Legacy locomotives on the track like conventional locomotives, as long as there is no command signal.
I do not recommend running Legacy and DCC locomotives on the same track at the same time. Can it be done? Yes, but there are issues. DCC tends to wash out the Legacy command signal, so the Legacy locomotives will not see it.
You should be able to convert an DC motored locomotive to DCC, I use the ESU LokSound L or XL decoders for that. I do not know if there is a way to convert AC motored locomotives to DCC. You can convert both AC and DC motored locomotives to TMCC.
On my layout I have no shorting switches to select between DCC and TMCC/Legacy. Because I have a newer Lionel ZW transformer, I can control it's voltage output with the TMCC/Legacy handheld so that is how I run my conventional locomotives that are not upgraded to DCC or TMCC.
I hope this helps you some.
