I think you could not rush the wedding. NRE (new relationship energy) lasts 6-24 mos. You have been together only 18 months and are making big decisions already.
I don't get why it can't be living together WITHOUT being married to see how you even work as roomies first. You could spend 1-2 years doing that first, to assess compatibility.
If you are going to go ahead with legal marriage, talk to a lawyer about prenups, what your will is going to be now with a legal spouse, etc.
Rent or buy a place with a floor plan that allows for separate bedrooms. You can still sometimes all share a bed if you want, by taking turns hosting, but you'd ALSO all have your own spaces, your own closets, space for your desks for your remote work, etc.
Keep your own banking accounts. It's ok to contribute a percentage to a joint account, for making house things like buying groceries and paying rent easier, but
maintain your own banking and money. Do not merge everything. All of you should have separate savings accounts, so if things go wahoonie, you can just get out and not be trapped living with exes due to finances.
With three people, it would be great to have three separate bathrooms. But it's livable with two bathrooms.
The best floorplans I've lived in were a 4 bed/3 bath for 3 people, and a 2 bed/2 bath for 2 people. I miss the ones that let everyone have their own bedroom and bathroom!
Use whatever terms you want.
It's not their biz. But be prepared for haters, cuz haters gonna hate.
WAIT. Getting married and living together, buying a house and sharing some finances are enough big changes. Do not add kids to the mix too soon, because if things do not pan out, it's easier to break up and divorce and never deal with each other again if there are no kids involved, just property to split up. Once kids are in the mix, you are stuck coparenting until the children are of age, and even then, stuck with seeing your exes at the children's graduations, birthdays, weddings, seeing the grandkids eventually, etc. There's a certain level of "basic polite" manners needed to not make your children's lives hell if you are all going to attend their milestone moments, or even to make plans to take turns attending and split them up.
What about the other two? What research and thinking have they been doing?
Will you all be taking a marriage prep class, online or otherwise?
Would you work with a poly counselor to fill in the bits unique to a triad marriage that conventional marriage prep doesn't cover?
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Galagirl