I've had minimal trouble with this fight myself, despite doing it with ONE character (yep, I handle it before I get anyone else on board).
What I do is simple: I use my bow to shoot them, generally the first shot from stealth, and as they approach, I move backwards, kiting them (and they're always dead before I reach Shadowheart). Of course my Tav does have a few damage bonuses to her bow due to how she's statted out, but it still should be manageable with a bow and one character. I'd say six rounds tops to kill all three for me personally, maybe nine without Tav having an archery damage bonus.
