With the use of a sin/cosine tables the triangulation calculations could be done faster than orders can be issued. We are talking about minutes of delay in a game with a resolution of 1hour.
Triangulation was the standard method of finding out where all radio signals originated from so listening posts were very good at it.
The results of these radio "sightings" will still not matter as much in HoI3 as it did in the real war. UK would give the convoy in the area of sighting a report that they have been spotted, and if in range they could send coastal command aircrafts to attack stalking sub/subs.
The convoy would be ready for battle the following nights, It could be reinforced by nearby ships and aircrafts exc.
Most of this will probably be simulated by naval doctrines instead of listening posts however since thats what made it effective. The knowledge and experience of how to use triangulation. The experience also tells how easy it is to find subs, just look around the convoys
Since uboats have so low visibility and sightings are not binary anymore I doubt we need to worry about uboats beeing intercepted by anything except air before they try to attack convoys.