I'm wondering if three terminals i.e. T5, expanded T1/2 and a new T6 could handle all the pax from 3 runways...?
No, not according to the projections contained in Heathrow's proposals:
"Terminal 2 (c55-60mppa) – terminal building is extended to the north of the Phase 1 building currently under construction, serving both the eastern apron within the current airport boundary and a redeveloped rectilinear apron on the old Terminal 3 site
Terminal 4 (c10mppa) – continues to operate as it does today.
Terminal 5 (c30-35mppa) – continues to serve the existing Terminal 5 apron and piers (T5B and T5C)
Terminal 6 (c20-25mppa) – a new terminal building to the west of Terminal 5 to serve the new north-west apron"