NEVER short pin3 to pin1 on a balanced XLR output.
This would have been a perfect topic to post in the "pro audio" section.
First let's explain the XLR pins :
1 : ground
2 : hot (+) normal signal
3 : cold (-) inverted signal
A XLR output is always a male XLR and an XLR input is always a female XLR. An easy trick to remember this is just to think of how a microphone looks like : it has a pins -> male XLR and it is a audio source -> output
Although it is not recommended you can make adapter cables from RCA to XLR and vice versa.
The rules are simple :
- RCA ground to XLR ground (pin 1)
- RCA center (non-inverted signal) to XLR hot (pin 2)
- on the MALE XLR cable part ONLY!!! Short cold (pin 3) to ground (pin 1)
The short is to avoid unwanted signals into pin 3. If you leave it floating it can cause unpredictable input (noise).
Pin 3 on the female XLR of a XLR to RCA cable is left floating otherwise you would short the output and risk damage (you will "overload" the output). Balanced output stages are mostly opamps (some use tranformers) and nearly all of them are protected against shorts, but protections can fail and overloading of the cold(-) signal can negatively influence the hot(+) output signal.
Leaving an output floating is not problem and if you think it is I have one question : do you shorten all the unused RCA outputs on your devices? I guess not.
PS : normally you don't connect XLR chassis to GND (pin 1) but in case you have any hum (ground loop) try connecting those. Some professional devices (especially amplifiers) have a switch for that purpose (ground lift, which factory defaults on "lift").
