XLR to 1x RCA

When connecting a 3-pin XLR to one RCA, you use the same wiring as if you were connecting an XLR to a 1/4" jack.
The -ve and shield of the XLR are joined together, either at the XLR end or the RCA end. The easiest way is to solder a link between pins 1 and 3 (shield and -ve) of the XLR, rather than trying to solder the shield and -ve wire to the sleeve contact of the RCA.

This produces an unbalanced audio cable.