I haven't read all the above responses, but coming from a planning angle being involved with same, three platforms/lines are a must at that location. Yes, there was (in my time) and likely so now, some overlapping of arriving and departure times of Down trains, which in peak times particularly, would be seriously be impacted without access to a second DOWN line platform between Fratton East and Portsmouth and Southsea.
But also in peaks times (evening peak specifically) not every train could run through to P&S or PH due to lack of paths, and so a termination had to take place at Fratton.
The empty stock would then pull forward from platform 3 and reverse south of the station, and run in to Fratton depot, these moves effectively blocking that particular line to Portsmouth for the duration of that move. Had that been the only DOWN line available to passenger trains towards Portsmouth & S etc, that would have had a further negative impact on the overall number of trains in the Southbound timetable towards P&S/PH.
A PS:
For those who know the Fratton area, there is a fourth road between Fratton (Depot) and Portsmouth and S, under the wall, known as the 'Back Road'.
This was/is used for ECS (empty stock moves) between Fratton depot and Portsmouth & S station/P&S Down Carr sidings/Portsmouth Harbour and vice versa, but there is no physical connection to same from Fratton station platform lines.