The story of these three men is found in Chapter 7.
In that chapter, Winston finds a picture of the three men from a newspaper. The picture seems to just be stuck in with some stuff he needs to look at for work.
The picture is important because it proves that the Party was lying about something. This would be a huge deal if people knew about it. These three men had been put on trial and forced to confess to various crimes. But this picture shows that their confession could not have been true. It shows that they were in New York on a day that they swore they were off in Eurasia plotting against Oceania.
It was a half-page torn out of The Times of about ten years earlier -- the top half of the page, so that it included the date -- and it contained a photograph of the delegates at some Party function in New York. Prominent in the middle of the group were Jones, Aaronson, and Rutherford. There was no mistaking them, in any case their names were in the caption at the bottom. The point was that at both trials all three men had confessed that on that date they had been on Eurasian soil. They had flown from a secret airfield in Canada to a rendezvous somewhere in Siberia, and had conferred with members of the Eurasian General Staff, to whom they had betrayed important military secrets.