What you are sizing the wire for is the least amount of voltage drop for the distance the wire needs to travel. Here is one online calculator:
voltage drop calculatorOf course, the wire needs to be large enough to handle the amount of current involved (amperage) but two or three small solar panels are easy to accommodate. You don't state whether these three panels will be connected in parallel or series but with a typical parallel connection having less than 25' of wire, 10 AWG should be fine.
I have a manual trip circuit breaker between the panels and the controller. It allows for quick and easy interruption of power between the panels and controller. There may be a time where you want to cut off solar power and the breaker is easier than shading the panels.