Legacy and TMCC 3 trains at once

There really is no reason in the TMCC / Legacy world that this doesn't work.  It is mainly an electrical world question.  As long as all the power sources are in phase and set at the same voltage output other than roller gap issues (if any) there is no reason why you can't do this.

I ditto Marty, I have three separate power supplies and have no problem moving from one to another, but all are phased.

Ray

Our modular club layout has four PH180 transformers powering sections of the track.  We move seamlessly between them, you never notice the transition.

Thanks, guys. I have them all in phase. I'm really having fun operating my trains after a long period of building the layout to this point. I couldn't have done this without the help of so many of the people that offer help on this forum. I have been able to be exposed to new sources of parts and supplies from the sponsors.

Of course, there are other options.  If you are running all TMCC equipped engines, you don't need seperate blocks.  Just wire everything as one 'track'.   If you have three separate loops, with the intention of running a conventional engine once in a while, you can use a BPC and your two power sources and have the capability of switching all three of your loops from command to conventional.  Why keep your conventional guys on one loop only?  With a BPC they can go anywhere on the layout. Switch the 'tracks' to conventional, no need to take the command engines off the track, and you can run on all three loops.  And then switch back to command control on any of the three loops when desired.  I guess I'm assuming all three loops are connected.