Pretty much anything off of the GCD will be fired all at once, but in the order the macro lists them.
Well, unless wow has some hidden cooldown or prevents it from being used with another ability.

Anything on the GCD is also fired in the same order and all at once, but because of the GCD delay secondary things also with a cooldown are fired on later presses of the key.

It's generally a good idea to preface your big nuke with Purges.
This removes negating effects such as grounding totems, and softens the target up, potentially removing Kings, Fortitude, Mark etc.