In this LRB article, Frank Kermode answers:
It was in 1948 that he [Empson] first outlined the theory in a letter to his wife: Bloom would like to make love to Molly but hasn’t done so for ten years, since his first son died, though he is keen to have another child. If he could get Molly away from Boylan and ‘get her to bed with Stephen’ he thinks he could manage it provided Stephen preceded him – perhaps when Stephen returned to Eccles Street, as he promised. Joyce was apparently ‘shy’ about this bit of narrative, and hid the point from his readers. Not from Empson, however, who expounded it several times adding more and more detail in evidence: for example, in two successive issues of this journal [London Review of Books] in August and September 1982, and finally in the posthumous collection Using Biography. He reached a point where he could not believe an unprejudiced reader could help finding what Joyce had rather cravenly hidden; and in any case he would presumably have given up the hope of a triangular arrangement by the time he started Finnegans Wake. But we are to understand that his desire for it had been urgent, and Empson studies it with appropriate intensity: the triangular outcome is ‘amply foretold’.
Absurd as it seems, I forget;
For the vision of love that was pressing
And time has not falsified yet
Was always a love with three corners
I loved you in bed with young men,
Your arousers and foils and adorers
Who would yield to me then.