Sunder allows you to spend advantage to cause damage to a piece of equipment along with attacking. Does that mean I don't have to specifically call and target the item before the attack in question, but rather as a reaction from my dice result?

Let supposed I attack a trooper and succeed with 3 advantages, do I still hit the trooper as normal and then I can decide to take 3 advantage to sunder and item of my choice? Could I also choose multiple items on that target assuming I have enough advantages?

Cheers!

Yes, yes, and yes, as long as all items are wielded or in the open reasonably.

It does require you to have an item with the sunder quality.

Also it takes 4 sunder activations(each for 1 advantage) on a single item (can be over a number of hits) to take it through undamaged-minor-moderate-major-destroyed. Just like Critical Hits are a great way to give a Doctor something to do, Sunder gives the Mechanic of the party something extra