currently I have a 27" imAC with 2 24" monitors… works well
I just tried swapping out one of the 24" for my 42" TV (with HDMI)
The TV worked, but the iMac main screen and the other 24" mirrored each other,
Putting the 2nd 24" back (removing the TV) and it all reverted properly
Well, the first thing to do is to go into the Displays system preference and arrange the screens as you want them to, eg side by side, or whatever. I think the default is to mirror if you add a new display.