I am no experienced SH3 Kaleun, but in your situation, I wouldn't even bother. 3 torpedoes aren't enough for the target in question, and that would only result in an unavoidable very long (or short) cat and mouse game.
Personally, I think you were very lucky.
When I get down to 3 or less torpedoes I just go home.
Naturally, if a lone merchant crosses my path (or sails near), I graciously accept the offering.
