[Closed] gearing question

I,m just getting parts together for my whippet build and am unsure on a couple of things if anyone can assist

I am going to be using slx gearing and have bought a medium cage rear mech, does having a medium cage mean I can only run a double crankset?

Also i,m on the lookout for the slx crankset and want to know what the difference is between a double and a triple
Is the spacing between the gears the same , so is a double basically a triple but instead of a 3rd ring it has a bashguard instead?
Hope that all makes sense what i,m asking, it does to me :))