Sounds like nothing wrong, if I understand your question. A mixer adds its inputs. Of course you can’t get much more than 10 V out of the op amp, so with a third input, if all of them are 5 V, the output will be clipped. Assuming you don’t want clipping, you then have to back down on the three (or four, five, or six) attenuator pots. I like mixers that have a variable resistor in the first stage feedback for that reason, you can just decrease the gain on that stage instead of having to fiddle with all the input pots.
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