You cannot and should not be able to shoot a salvo, either with a fleet boat or a U-Boat. It was a grave mistake for SH3 to make available an option which simply was not an option in the real submarine. Now we have spoiled skippers everywhere complaining that the "feature" was nerfed. It needed to be nerfed.
Just as in the American navy, the German navy had a policy of a couple or three seconds between torpedo firings. This was for a very good reason, especially when magnetic pistols were engaged. What do you think torpedoes are made of? Hmmmmm? And what happens when they are close to each other? Do they know the difference between the magnetism of a target and the magnetism of another torpedo? What if there is actual physical contact between contact pistol torpedoes on the way to the target? A premature detonation of a single torpedo would explode the whole salvo!
BOOOOOOOOMMMMM!!!!!!! Here we are, come and kill us! We've shot our wad and made a great fireworks display for your entertainment. We can't hurt you now. Come and kill us!
No, both submarine forces had a strict policy of spacing torpedoes far enough apart (at least three seconds) to avoid interaction between torpedoes.