Loading a random scene/level.

Imagine I had 4 levels: MainLevel, Level1, Level2, Level3. I would like the player to enter MainLevel and after a set amount of time (seconds) either level 1,2 or 3 will be loaded at random.

I am aware of the "Application.Loadlevel" and "WaitForSeconds" scripts however I am unsure how I randomly select a level from a set group.

I'd like to be able to drag levels into bars in the inspector tab, that way, the script may be used on multiple times. I think this is set using String variables.

Not asking for a full script, just looking for some groundwork and a place to start.

Thank you for your help, I am new to coding. (Sorry mods' if this question has already been asked but I could not find the exact answer I needed.)