xul, I gotta admit I don't know what you are talking about. 3.6V/18A = .2 ohm resistance. I follow you up to then, but wire size should be 10 percent of that? 10 percent in what sort of measurement? Mils? AWG? Inches? MM?
The charts show to use 9 gauge or larger. If you wire a house, in the United States at least, you would pull 14 AWG Romex for 15A outlets, 12 AWG Romex for 20A outlets. For low voltage wiring you would have to worry about voltage drop so you would want to take wire impedance into account, but for a 3" run I would ignore that entirely. So, I would consider either 12 or 14 pretty much safe. That's only if the OP means a parallel circuit, which is open to debate from his specs.