Ah, yes Shed, the eternal B pier.
I suppose in terms of parking stands, treating it as an extension of T3 might make sense with perhaps a slightly shorter walk for pax than from T2.
However, what about check-in facilities. T3 in the morning peaks can be pretty congested, and even allowing for AA/KLM/AF moving out, and the small space where J class pax check in for the US flights, it surely wouldn't accommodate all of Jet2's ops at current levels. At present they have a very decent self-contained facility on ground level of T1 with its own security so I doubt they would want to be squashed into T3.
As I recall, B pier has some 14 stands for narrow bodied a/c so I was assuming that would really be an add-on for T2.