Probably obvious, but sort of struggling figuring out some basic conditionals...

I have the statement below to add a bit of code to only my forum index:

<if condition="THIS_SCRIPT == 'index'">
My question is how would I write two conditional statements... One that would display on the index and forumdisplay pages only, and another to display on the index, forumdisplay and showthread pages (with showthread only applying to specific forums)?

Really appreciate any pointers.

Thank you.