DCC is a control system. It is not tied to any scale or number of rails. You just select your loco receivers (decoders) based on the amperage draw of you locomotive. You can little tiny ones rated at .5 amps that will fit in an N scale loco and large ones rated at 8 amps that will fit in an LGB size loco. Most of the decoders are rated 2-4 amps. 2 amps is generally considered plenty for HO. Many older O scale locos draw 2.5-3.5 amps so you need a 4 amp decoder. I think a lot of the newer stuff in O draws less than 2 amps under load.
I know for sure of one using DCC in 3 rail and has been for years. He is a member of my round robin group and models the Western Maryland. I think I have heard of another loco guy doing 3 rail DCC but not sure.
