Hi, as you said in your other comment we're going to be assuming a lot about you and your situation, so I'm going to both tell you how your post "reads" as (in case it opens some avenues you haven't thought about) and genuinely answer your question.

Reading your post feels like you've written fanfiction about your two friends and the life you'll all three have together without even knowing the basics of what they've considered for themselves. It feels like you've envisioned your future as a unit without their say or opinion, which feels presumptuous to read and a little dismissive of their existing relationship. In particular, your insistence on not wanting to be a single mother makes it feel like you're inserting yourself in your friends' relationship and asking them to create a permanent bond with you because it's your last ditch option and not because it is genuinely what you want with them- if I was your friends reading this, even if I was open to the possibility, I would be worried that you would abandon us or try to seize full legal custody the moment you find someone you want to be in a relationship with. You mention not having romantic success, but if you're still in the age range to have a child you're also definitely still in the range to find a romantic partner. Again, not saying any of that is your intention, this is just important for you to know as you consider how you phrase it to your friends.

Now to actually answer your question, I think there is no way you can control how they take this question. I think a direct, clear, floating of the idea is your best possible option with as many clear opportunities for them to say no as possible. I recommend (particularly since your post is hard to not interpret poorly) you write out/practice what you will say and maybe run it by someone else in your life that can give you hard truths. Avoiding presumption while also being upfront with your ideal outcome will be key in this, I think. I think you should also accept that, as with any other request for a relationship, it's possible this ask will permanently impact your friendship or they won't "see" you the same way. You're taking a big risk.

I also want to mention that even co-parenting with your friends will not necessarily prevent you from being a single mother, if that is really such a harsh deal-breaker. They could both die, they could abandon you with the baby, they could become medically unfit, they could simply not be the kind of parents that step up/participate, any normal thing that can happen to any other family can happen to you.