Should I just be chill and let it play out, or should I bring up the idea of a poly relationship myself?
Let's keep it simple.
Do you want to wait and see how it unfolds or do you want some clues?
That's only something you can decide. Some people like the not knowing and the unfolding in a mysterious and exciting way where they do not know where it will end up at or what choices there may be on the table. Some people want to know the potential
to unfold to a certain place is at least on the table as a "maybe." Because if it is not even on the table why bother? They want an unfolding that at least has a chance to go to a wanted destination. It's worth the trip to them to find out if it reaches there or not.
Are you interested in a triad with them?
Sounds like you are. You could ask them -- "What is this we have here? Is this just friends with threesome sex benefits and that is all it is? Or is friends with threesome sex benefits with a potential to grow out to a triad situation? How do you all see it?"
And then see what the answers from each one are. Go from there and determine if they want similar things that you want too. Then you can decide for yourself if this is going to be worth further investment of your time or not.
When you say you are afraid to bring it up directly -- what does that fear speak to? What do you think would happen that is scary to you?