I would have thought pin 2 for positive and pin 3 for RCA ground and leave pin 1 open or connected to the cable screen
When using single-ended equipment, you need a signal and a reference against which the signal is measured. If you would take pin 2 and 3 then the reference would be out of phase instead of zero. It might produce sound but this is not how it should be done.
There are two connection schemes.
Regular connection, used these days by most, but not all, is as follows:
Pin 1 - earth/ground
Pin 2 - positive signal
Pin 3 - inverted signal
In the early days, Jeff Rowland and Cardas (and some other brands) used the following:
Pin 1 - earth/ground
Pin 2 - inverted
Pin 3 - positive
When connecting an XLR cable to an unbalanced input, you should take the positive signal half and use the neutral/ground (always pin 1) for reference. The inverted signal half will then not be needed.
Whether one needs female or male RCA connections on the adapter depends on whether you want to convert your preamp XLR outputs to single-ended and then use a single-ended cable, or convert a balanced cable to single-ended. Either way, you will need XLR to RCA adapters. In the first case, you will need a female RCA connection (into which you can plug an RCA-RCA cable), in the latter case, you will need a male RCA connection (that plugs into a female RCA input).