To connect microphones to microphone preamplifiers
Yes, XLR is a balanced.
3 pin XLR's are mono. 5 pin XLR's are special connectors usually used in stereo microphones, but usually split on the other end to two mono XLR's.
Between microphone and microphone preamplifier
Discounts for XLR cables can be found on Monoprice, Crutchfield, Musicians Friend, ShowMeCables, zZounds, Amazon, Mobile Beat, B&H Photo-Video-Audio, and All Express.
A microphone preamp's output may be XLR or 1/4". You will need a cable that will adapt that output to a 1/8" stereo plug for the line input of your computer.
To connect an RCA cable to an XLR cable, you must solder links one and three from the XLR together, and connect those two wires to the end of the RCA component, along with the second wire. It is very useful to use a diagram to make sure you connect the correct cables.
You solder ground to pin 1, hot/plus to pin 2, cold/minus to pin 3
Search for XLR splitter
It's also called XLR. If it has pins, it's XLR male, if it has holes, it's XLR female.
XLR is a Cadillac
As in any live recording applications, the more length you add to cables, the more noise that cable tends to pick up. Try to keep cable lengths to a minimum. If that is not possible, give it a shot and see how the recording sounds. You may want to try an XLR microphone application as XLR cable are shielded to reject unwanted noise*. *XLR cables may still pick up unwanted noise when running them at long lengths.
An XLR to USB cable has a converter in it. It isn't just a cable. An audio interface can be used to allow you to use any dynamic microphone or instrument and connect it to the USB or Firewire interface with an XLR cable.