The original plan was the 6 star-shaped runways on the present site, and another three proposed north of the A4, which of course were never built. The 6 runways was not completed for some years, followed shortly afterwards by starting to close them off for central area buildings, so the full set didn't last long.
The tunnels opened in 1955 along with T2 (then the Queens Building) and were originally the current dual carriageway, plus the two small tunnels either side now used for cars (provided you don't have a big roofrack), but one was originally for pedestrians and the other for bicycles. T3 (then the Oceanic Building) opened in 1962. Until these openings the relevant flights (European and Intercontinental respectively) operated from the north side, strung out along the A4.