ouch... those are some sick fish!
the 3 stripe damsel definitely has a lot of finrot on the caudal fin. tons of ich, too. i guess the question is whether you want to save them or euthanize them to prevent infection of other fish. they definitely need to be quarantined. what's stressing them out so badly that they're so sick?! if it's nothing you can see, it might be poor water quality (brand new tank?). you'd definitely be best off getting rid of the cause before you think about getting any other fish.
i haven't been in saltwater for much more than a couple years (most of my fish experience is from freshwater tanks and ponds), so can the common fish bacterial and fungal infections pose threats to inverts? Disregard that something in the tank is seriously out of balance. Let's assume you bring home a fish that you didn't inspect thoroughly, so you missed a couple ich spots until after you already added him to the tank. any ideas/experiences?