Connecting multiple monitor speakers

I’ve been the using the UR44 for a while now and like it very much. But I’m puzzled regarding adding a third set of monitor speakers.

So my main studio monitors are plugged into “main output L/R” and connected to device port UR44 Mix 1L & UR44 Mix 1R in the control room. A second set of speakers are connected to Line Output 3/4 with a TRS jack and connected to device port UR44 Mix 2L & UR44 Mix 2R in the control room. All works great, I can switch back and forth between speakers A and B in the control room. This is how I’ve been doing it forever.

Now, wanting to add a third set of speakers, I added another set of monitors in the control room and plugged them in Line Input 1/2 in the UR44 (again with a TRS jack). But there’s only two mix buses in the control room, so how to connect?

Consulting the manual, I read where the Main Outputs and Line Outputs 1/2 both output the Mix 1 signal. Indeed, when I play a mix with “A” selected in the control room, both sets of speakers (the main monitors and Line Input 1/2 speakers) output the sound. All I could figure to do is to turn down the mains to listen to Line 1/2 speakers, and, conversely, turn off the Line 1/2 speakers and turn up the mains to hear them.

While this does allow me to monitor through three different sets of speakers, I’m thinking there must be a better way. For one thing, doing it my way precludes the necessity of adding a third monitor in the control room, since two sets of speakers are playing through monitor 1. This doesn’t seem right.

So…how the heck can I switch between three sets of speakers by just clicking A, B, or C in the control room??