How can I anonymously select players for a role?

I want to make a game similar to murder mystery 2, I can already do this using teams but that is not what I want.

I want a script that selects one out of three roles for the player. It's very simple.

Bonus

I want a script that not only selects one out of three roles for the player, but I also want an if statement script that works inside a brick to...

allow the player with a specific role to access a gui that other players can't access (ontouch script)