Quoting myself from another thread... a little obnoxious, but I wanted to explore this further and was getting way off topic:
"I truly believe that humans just act differently in dyad situations, in ways that are both revealing and bonding."
My point with this statement was, at heart, to reinforce the old idea that a triad is not one relationship of three people but rather four relationships -- A&B, B&C, A&C and A&B&C. To focus on only the last one of the four is, to my mind, to misunderstand how human relationships work.
Editing in a caveat: I absolutely do not mean to imply that real and significant intimacy and bonding can't occur when all three people in a given three-person relationship are present, both on the dyad level and the triad level. I've very much seen that that isn't true.
Rather, I think it's important to recognize that the dyad pairs within the triad do exist, and to give them their own space to flourish from time to time.