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
Is that NORMAL?
No idea, but have you changed the setup in the Displays control panel?
alll I did was unplug the HDMI cable from the 2nd 24" monitior and plug it into the TV
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.
What resolution are the TV and the 24" displays?
Was it powered on at the time?
The mac assumes any new monitors are going to be mirrored. You will need to realign your screens.