Maybe I do not understand your question, but with the stock of sh3 when selecting a tube and you open it, switching tube with the "Y" (or click on another tube) has the effect of closing the previous one. But if you like you can do the following:
1. - Look for the "Commands_en.cfg".
2. - Open it and look for the following entries:
[Cmd309]
Name = Close_torpedo_tube
Ctxt = 1
[Cmd312]
Name = Toggle_open_close_torpedo_tube
Ctxt = 1
3. - Now, these should look like:
; [Cmd309]
, Name = Close_torpedo_tube
; Ctxt = 1
; [Cmd312]
, Name = Toggle_open_close_torpedo_tube
; Ctxt = 1
So you can have open several tubes without having to fire a salvo ... I do not know if this is realistic, commanders always fired a torpedo or a salvo. The idea seems to be of makman94.
Good Hunting.