Welcome to our forum.
You are in a "V" situation: two "arms" (your girlfriends) and one "hinge" (that would be yourself). You have a lot of responsibility on your shoulders as you must divide your time and attention equitably between two partners.
I would say that you have the option to opt out of marriage altogether, but it it ultimately up to you (and your two partners). Responsibility for the children would be based on who the parents are, but I'd assume you'd want to take responsibility for any children you had fathered.
There's also the questions of whether the three of you will live separately or in the same home together. A lot of this depends on what the women want, or if they even want to share a domicile or realistically can.
I wouldn't try to rush into any of these decisions. I would give the V time to stabilize. It would help if the women spent time together developing a platonic friendship, but if they don't want to do that, there's nothing you can do about it.
I would suggest trying to improve your relationship with each woman individually. It's not all about what they can offer you, it's about what you can offer each of them. Find out how you can do that. Get some communication going.
I would also try reading many of the threads on this site, and posting your thoughts, questions, and concerns. There is a lot you can learn here; it is an opportunity.
I hope things work out in your relationships.
Love means never having to say, "Put down that meat cleaver!"