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) plug.
The positive 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 negative) of the XLR, rather than trying to solder the shield and negative wire to the sleeve contact of the RCA.
This produces an unbalanced audio cable.
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