I tried this years ago in SH2 but it didn't work, but in SH3 it does. I created a small British convoy and attached a German wolfpack using Serg's AI subs.
I placed them all around the convoy as you can see from the screenshot, when they came in range they commenced firing, I placed myself the player sub in the middle of the convoy as a British sub so I could watch the action.
Also in the action I got a screenshot of a DD ramming a sub, and as you all know they both died when this happens.