The reason for selecting conf3 in the MCDU is one of approach speed.
If the MCDU is left at Conf Full the speed that will be used during approach is F speed. Normally this is of no consequence if there are increments to be added to VLS as the managed speed will then be above F speed anyway.
However if the a/c is light and there is no ( or a small ) increment to VLS then the a/c will fly at F speed and not Vapp as it should.
To fix this the FCOM now calls for F3 to be selected in the MCDU to tell the system that we are going to fly the approach at flap3 and thus reduce to Vapp when F3 is selected. This also gives us the benefit of having GS mini active during the approach.
This all gets a little confusing however when you realise that we will mostly be flying selected speed for approaches during flap/slat malfunctions where the majority of the F3 abnormals occur! FCOM 3.2.10 p7 and QRH 2.32 refer.