Gauge sizes in copper electrical wire will get larger as the number gets smaller which method 10 AWG has more copper in it than 12 AWG. It’s very doubtful to find a supplier with stock on any strange gauge sizes such as 11 or 7 because the difference is so minor that the application could easily use the size above or below the strange size needed. However, when the gauge sizes get larger there’s much more room in between each size which is why strange sizes start to be included at 3 AWG electrical wire.
The outer diameter of 14 AWG bare copper is .064 inches compared to the next size down of 12 AWG at.081 inches. That’s a difference of .017 inches which method a 13 wire would be nearly useless to have around. Now, the difference between a 4 wire (.204″) and a 2 wire (.257″) is .053 inches putting a 3 AWG electrical wire directly in the middle at .229 inches. There is also a 1 AWG wire included as the sizes get larger for the same reason.
A 3 AWG electrical wire is most commonly found in the THHN THWN family. THHN stands for Thermoplastic High Heat Nylon and the “W” in THWN stands for Water resistant. Both approvals are written on the insulation of all THHN copper wire nowadays because the cost of manufacturing both types got out of hand. The cost difference between the two was minor so combining them was a good idea.
This kind of copper electrical wire is most commonly used in homes and building by installers, electricians and contractors who pretty much just say “I need number 4 wire” and it’s understood by the supplier that they need THHN wire. There is a cost savings for the contractor when they can use 3 AWG wire instead of 2 AWG wire because the cost is directly pushed by the amount of copper used to manufacture it. I assume it goes without saying that a 2 AWG wire has more copper than a 3 AWG wire so it makes sense for the contractor to find the 3 AWG wire instead of just purchasing the number 2 before looking around.
Speak to your electrician about the size of copper you’ll need and speak to your supplier about the kind of insulation you’ll need based on the application. Suppliers are trained more to determine which kind of insulation protects best against indoor, outdoor and underground conditions.