If you still have three free input channels, it is simple:
Feed the talkback mic (let’s say socket 44) to all three channels (say #42,43,44), send #42 to the IEMs, #43 to the manager and #44 to the techs. You could even eq/comp/level them differently.
Now you can assign the first three softkeys to mute these three channels separately.
Without having to watch the inputs themselves, you’ll always have a visual feedback via the softkeys’ LEDs about each talkback’s on/off status.
If you only have two free channels, use the regular feedback function above the softkeys to feed one of the three destinations.
🙂