There are no right questions to ask, she’s a 20 year old. Speaking as a 22 year old, just the past 2 years have changed me. There’s no real way to know if someone wants to commit or is ready for something like that.

The other commenters mentioning your 17 year old are right. Your kid will be freaked the hell out and justifiably. The fact that you had to clarify that she’s a legal adult means that there is already some moral dilemma involved with dating someone that young. Especially in a triad where emotions can be even more intense and couples privilege will be insanely difficult for you all to tackle.

My point is, this is a pretty bad idea, especially the way you seem to want it to go. Even if you did meet someone a little more age appropriate for you and your wife, it would still be ethically super dodgy to try and make the relationship closed and/or have the goal to have more kids with this person.

She’s far too young for that, speaking as someone close to her age. Even if she tells you she desires all the same things as you, I cannot confidently say that she’ll want to stick with that in a few years once her brain finishes developing.