The extended vertical fin fairing is Airbus’s response to the design department’s decision to remove the vertical stabilisers on the T3 variant. The first T3s, without the fin extension, had (still have) an uncomfortable tendency to fishtail at high speed, especially in turbulent conditions. The fin extension is now offered as an optional refit for those early airframes. Makes you wonder how come the test pilots signed it off in the first place.
This tendency only became apparent when the Helionix model (P3H and T3H) was released which has a completely different AFCS system to the early T3 and P3.