Everything you have described so far points to a routing issue where Mix3 really isn’t being outputted where you think it is and instead one of the other mixes (or outputs) is assigned to the physical outputs you “think” should be Mix3. It is possible to output something to more than one physical output. For example, it might be as simple as Mix2 being routed to both the Mix2 output AND the Mix3 output.
There are a couple of ways to troubleshoot this. The easiest is probably to talk into a vocal mic while you listen to Mix3. While talking, bring down that vocal mic’s channel in Mix1 mix, then try Mix2, then Mix3, then Mix4, then the Main Mix, etc until you hear the change in Mix3. Whatever mix you are controlling when you hear the change is the mix that is being routed to the outputs you “think” are Mix3. Odds are you will find that the routing has been duplicated and you aren’t actually outputting Mix3 on the physical outputs you are using for Mix3.
If this doesn’t work, then I would send a signal out to only Mix3 and remove it from every other aux, output and matrix. Then confirm that you can hear this source in Mix 3’s output. If you don’t, then this confirms that something being patched incorrectly and you really aren’t hearing Mix 3 out of the Mix3 output. If you can hear it, then it would seem things are routed correctly, but some setting is incorrect in the signal chain which is preventing it from working as expected. At least you will know where to look (routing page vs Mix3 signal chain) based on this test.
