The passage from "The Waste land" is a reference to Christ's journey to Emmaus following his resurrection, where he joins two disciples that do not recognize him: "Who is the third who walks always beside you?" one disciple asks the other.
Eliot's notes on those lines are for once somewhat helpful:
"The following lines were stimulated by the account of one of the Antarctic expeditions (I forget which, but I think one of Shackleton's): it was related that the party of explorers, at the extremity of their strength, had the constant delusion that there was one more member than could actually be counted. "
Perhaps we need a catchy zither theme from "Shackleton!" the musical. Sir Andrew should get off his duff, cancel any further "American Idol" embarrassments, and get on the stick, pronto. Before the gout incapacitates him completely.
I imagine that the use of fevered dream sequences by trapped crew members should afford him considerable liberties with the script, up to and including the incorporation of mid-European Ferris wheels.