I noticed several "triads" in this novel, one in each section, which I think is Asimov speaking to us about the importance of relationships and also a structural choice. Foremost is the Odeen - Dua - Tritt triad in the paraverse, obviously. But, there was also the broken relationship between Hallam, Lamont, and Bronowski in the first part and the emerging triad of Denison, Selene, and Neville in the third.
As Glenn pointed out, there is a very direct comparison between Dua and Selene, but I think there are also good comparisons between Odeen/Denison (the rationals) and Tritt/Neville (the parentals). Denison is thinking his way out of the imminent issue with the Pump, but is only successful with Selene's help (reflecting the Odeen/Dua effort). Neville seems to see himself as a parental for the whole lunar society, interested in the safety and continuity of the group.
Maybe Hallam/Lamont/Bronowski aren't a triad, but a group of Hard Ones. They've reached a level of maturation, but also inability to change.