How does wire bundling with the same AWG work?
Ampacity for transmission is based on temperature rise.
- For short lengths you can exceed this, but you may be more interested in % voltage drop which is a function of both length and current.


First decide what voltage drop in connectors and wires is tolerable.  (e.g. 1%)
Then determine length of cable x2 for (supply+return)
Then compute max resistance R=Delta V/I 
e.g. 0.12V/3A= 400 mOhms then divide by length to get  Ohms per unit length and choose proper wire gauge or bundle.


ref http://www.powerstream.com/Wire_Size.htm

For 1m length cable 0.4 Ohm/m reads AWG29 as smallest size. So if you are ok with 1% loss and 3A is peak and not average.  So AWG20 is great for short lengths.