I have two partners (I’m the point of a V) and with partner A, I can see us settling down and getting married and all that, but with partner B, I can see us dating for a long time, but not necessarily settling down. Is this a common feeling?
It’s your feeling, you’re having it, and it’s real. It doesn’t matter if it’s a common feeling; there is no bell curve of frequency that determines whether a feeling is appropriate or valid.
As long as this arrangement is working for you and your partners, it’s all good. Does partner B know you feel this way? Or are they expecting/hoping/looking ahead to settling down with you in the future? Do you and your partners use a tiered (primary/secondary/etc) system for polyamory, or are you just letting the relationships define themselves?
As long as everyone is on the same page and expectations are shared, it’s all good. Don’t set someone up to be hurt or let down; don’t pretend like your feelings for B are something other than they are to keep B on the line. How common this feeling is is less important than whether everyone involved is okay. Do what’s right, healthy, and fulfilling for all parties involved - even if you three are the only people on the planet facing this issue.
(And to answer your question, sure, that’s a common feeling. Relationships are different; people are different; it’s normal within poly to experience affection, commitment, etc. slightly differently across different partners.)