Based on reading multiple posts here I would say that it is VERY common.
The "equilateral triangle" ideal that many people starting out with poly are looking for is, I have observed, not very common. Many people struggle with this as their triangle turns into a Vee or lopsided triangle because they feel that the "only" way things are "supposed" to work is if everything is equal.
BUT every relationship is different, because people are all different. Relationships grow and develop (or don't) at different paces. And that is OK. Instead of focusing on their relationship and what is (or is not happening) it might be helpful to put your energy into looking at your relationships with each of them and see if you are getting (and giving) your needs and wants met (and likewise for them) within the confines of THAT relationship. It is up to them to work on getting their needs and wants met in their relationship.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (22+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe