The question isn't clear. 3 amps could be persuaded to run in a tiny wire, or a fat one. The current doesn't tell you anything about the size of the wire.Are you asking how to decide what size wire you need to carry a particular current?
Hmm - based on refs in this:https://wiki.xtronics.com/index.php/Wire-Gauge_AmpacityFirst we'll rearrange things to solve for A:A = (p / R) * lThen note - Ohm's law - V = I * R or R = V / I - so sub that in place of R:A = (p / (V / I)) * lWhich I think can be re-arranged (math ain't my strong point!) to:A = (p * I * l) / VWhere:A = cross-sectional area of the specimen (measured in square meters, m²)p = static resistivity (measured in ohm meters, Ω-m)I = current (amps)l = length of the piece of material (measured in meters, m)V = voltagePlease cross-check my math, of course - it's probably wrong! At any rate, I think between the link above, and the following (or something similar):http://www.powerstream.com/Wire_Size.htm...you'll probably have enough info to figure it out.