The number of talkgroups you're monitoring isn't the problem. The radio has to:
- Scan to find the control channel and lock (this step is slow)
- Once locked, see if any selected talkgroups are actually talking (this step is pretty fast-- maybe only 1-2 seconds, even if you have a bunch of talkgroups)
- Move to the next system & repeat #1 and #2.
While it's doing steps 1 and 2, the other two systems you are listening to are NOT being checked at all. That's why they are telling you you will miss transmissions.
You will. Same problem exists on Motorola's top-of-the-line APX radios when you try multi-system scan.