You really got a good deal on that!

When you double the wire you effectively double it's size in diameter (but not the AWG). So if you look on the wire size chart at the site below and double the diameter for 18 AWG it shows that it's about a 12 AWG wire at that point.

Losses will be minimal at 3A only, at 12 AWG. If you are going to go through the trouble of running wire you might want to plan for the future and run as many as you can.

This is a great resource to have bookmarked, I've gone to this site many times.

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies
http://www.powerstream.com/Wire_Size.htm

"in American wire gage every 6 gauge decrease gives a doubling of the wire diameter"