I see some sound advice here already but it leaves me at a point of wondering if we've even maybe jumped "ahead" ?
Something to really focus on I think is that overall awareness of "polyamory" in the general population is pretty low and even confused in some who have at least some minimal exposure.
And it seems from your writing you may be in that position right now and IF that's the case then I feel you need to back up, slow down, and as some have suggested start ALL having some deep discussion on the topic & possibilities in general. If you go rushing ahead you may tank the ship for lack of a good compass and understanding of wave behavior.
The general concept of being able to be -successfully - in a loving relationship (let's say 'romantic' although I hate that term) with more than a single individual is NOT something most people are exposed to in our culture.
For most it comes as one of those "WHOAAA - you are saying WHAT ???" moments. And THAT is where the learning process starts.
There's LOTS of good resources out there including forums like this that you all should probably investigate. A couple that come to mind are......
The folks here I know will be happy to share their views & experiences with all of you too.
Good luck !