The manual of my DAC warns against running the unit's balanced XLR outs into unbalanced targets with cables whose pin 3 ground is connected. I have such cables and would like to run the XLR outs into my unbalanced speakers. Should I simple clip the pin 3 ground, or might that cause problems?
That depends. If you are running phantom power (or any other kind of power supply) over this line, do NOT connect it to ground directly. Connecting it to ground will effectively connect the power supply to ground, resulting in a lot of power being drawn (7mA in the case of a standards-compliant P48 phantom power supply, 18mA for P12). Therefore it's usually better to connect pin 3 to ground via a DC-blocking capacitor (22µF or lower should do).
However, it's probably safe to just leave pin 3 unconnected. I have it like this on most of my cables and I've never had a problem.