RCA to XLR interconnect wiring diagram

Caution against running the ground to pin 3. This shorts the negative pins of the right and left channels, which were never meant to be shorted together!
If you're lucky, this will only cause mild distortion...
Instead, run a resistor between pins 1 and 3. The value depends on the citcuit. Try 10K for transistor amps or 100K for tubes.

Going from balanced to RCA you would do just as you say. Terminate pin 3 to 1 with a resistor, depending on the the load at the destination (which "looks" from pins 2 to 1).
I'd try and match that load, so the balanced stage sees a balanced load. Not always a necessity, since some equipment just use an inverting buffer for the negative output...
With that kind of equipment, my question would be: why bother?! :-(