Using Arduino, how can I dynamically change signal input from one pin to another?
Considering it will most likely be composite video on these cameras, which are relatively low bandwidth (&lt;10 Mhz at best), any common bus switch or multiplexer would work.

You need a 3 to 1 or a 4 to 1 switch. Most have simple analog control, two pins that are high or low, so you will need two spare pins on your arduino. You could also find i2c or spi based switches instead, if you already have spi or i2c parts on the multirotor that you are using, and add them to the bus.