Would a 3-inch shell have enough explosive power to threaten a modern submarine?
Short answerThe suggested target is a small uncrewed underwater vessel, not a full-sized modern submarine.
NotesA 3-inch shell is not proposed here as a weapon against a full-sized modern submarine. The possible target is a much smaller uncrewed underwater vessel. A depth-charge-like shell is imagined as a way to put many small charges into an area quickly. The example suggests roughly a hundred in a minute, as an illustration of concentrated fire rather than evidence of a proven engagement.