Congrats on your relationships. It sounds like it is off to a good start.
I would think it would be easier for three to come together rather than a couple adding a third. That gets rid of someone feeling like the third wheel to an established couple. From what I have seen, triads are not the easiest to keep stable. As people change over time, it tugs at relationships. It is hard to get it to tug well in a triangle type relationship. So I think it takes some additional work to keep a triad going well.
But in reality, this is true of all relationships. Imagine a standard statistical marriage with a 50% success rate. Now imagine tha a triad is made of three couplings (or bonds). If they had a 50% success rate, then there is a 1 in 8 of it working, 1 in 8 of it fullly failing and 3 in 4 of it turning into a vee. (Huge oversimplification.) Triads have many advantages like having additional support from other partner and some bonds strengthen the other bonds. So This improves the odds for triads.
But it all comes down to relationship skills you learn over time in a poly relationship. Just make sure everyone's desires and needs are communicated. Deal with jealousy more as an indication of deeper feeling than treating the symptom itself.
Sharing or not is an individual opinion. Do you worry about people judging you? Could it affect a job? Personally, I decided to come out of the closet on several issues. The end result is that I surround myself with friends who like me for who I am and I don't waste my time chatting with family members that don't like me for who I am. There is nothing to be shamed of. Most peope will just treat you like you bought a ticket to the Titanic and wait for the inevitable fallout. Or they may feel bad for you since they will see the guy has having a mini-harem. When I was in a triad, my gf was out to a few of her friends at work. She explained that she at least got the variety of a man and a woman.
There is no reason why you three could not get "married" though it will be more of legal issue in a realm dominated by monogamy. For example, the law will tend to see one couple with a roommate. It will also see two people as parents. Maybe one way to deal with that last part is to divide up child bearing in three ways. One be an egg donor and the other be the surrogate mother.
Wanyway, wish you well.