i recently came into possesion of some The Joker Thugs from the Batman set. they have a ability that says

ROLLING WITH THE PUNCHLINE: When Mastermind is used to deal damage to The Joker Thug reduce damage dealt by 1.

now if i can get them to pick up the iron man briefcase armor which would give them invul for a reduction of 3....is that legal to do? cause if so then im gonna love these thugs

From the Player's Guide:

If a character could use multiple game effects that would reduce or ignore damage dealt, only one of its game effect can activate, to be decided by the character‘s controlling player.

In other words no. You'd have to choose between reducing by 2 for Invul or reducing by 1 for their ability.