Three makes it a bit trickier but not bad. If you had two, you could use a 4PDT toggle switch which is really easy. On those, the center is where you connect the input and either side you just match up top to bottom and that is A/B. Just remember to put the left position outside on the right on the inside or it will appear backwards when you switch (this makes sense when you have it in front of you).
For a rotary switch, you have to think in terms of pieces of pie with the circle divided into four pieces of pie and the center being the input switching to each of the four areas. As the switch rotates from 1 to 2 to 3, it moves each of the four center connections to the corresponding connection, so all the #1s are on at once, or all the #2s are on, or all the #3s are on. Here is a diagram that helps you think about it graphically.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.hawthorneaudio.com%2Fphotos%2Falbums%2Fuserpics%2F11473%2Fnormal_4p3t_connection-1.jpg&hash=3384eceed9bcc510e60555846039615deb34f306)
Here is a rotary switch you could look at (others have been posted that are less expensive):
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CKC7008-NDI say go for it and post up if you have trouble figuring it out!
John Kessel
Hawthorne Audio AMT K2 Reference Speakers
Paramount 300B w/MQ All Nickel Iron, Mundorf S/G 5.5 uF, and Vcap Teflon .1 uF
Auralic Taurus Preamp/Auralic Vega DAC/Auralic Aries Streamer
and lots of room treatments!