To the best of my knowledge: No.
It may be as simple as wanting a clearly alien body shape. On Earth, animals invariably have two, four, six, or eight legs, so giving something three legs is an easy way to signal that it's not from our world.
It is suggested the Homomda were patrons/mentors to the Idirans. So there may be some connection between the species in the distant past. (The Homomda might similarly have influenced the Chelgrians, although I don't think this is made explicit.)
The Culture stories are vague on why there are so many humanoid species in the galaxy. An awful lot of them have the basic body plan of two arms, two legs, and one head with no explanation as to why they look like that. In the novella The State of the Art, a Culture ship visits Earth in the 1970s and most of the crew can circulate among humans with little or no alteration in their appearance.
I am fairly sure the reason for the plethora of two- and three-legged species in the Culture universe is not addressed anywhere in the stories. It may have been intentionally left as a Cosmic Mystery. Sadly, Iain M Banks is no longer around for us to ask, so it may have to remain so.
The essay A Few Brief Notes on the Culture further indicates Banks was deliberately leaving this unexplained:
...there are two untold stories implicit. One is the history of the Culture's formation, which was a lot less easy and more troubled than its later demeanour might lead one to expect, and the other is the story which answers the question; why were there all those so-similar humanoid species scattered around the galaxy in the first place?
Each story is too complicated to relate here.