The cage length is to minimize chain slap and for proper chain length.

As a *general rule* a medium cage is for a 2x setup - 2 chain rings up front.

A short cage is for a 1x setup - 1 chain ring up front.

If you at running 3 chain rings up front, you'll want a long cage.

You can more in depth - spread of cassette gears, etc - but this should at least get you headed in the right direction.