When Auto and I started dating, it began as a few casual "get to know you" dates. Then she messaged me and just said, "I'm interested in you and I'd like to ask you out on an official date." From there, it just grew organically.
My advice is not to push anything, just start acting like you're "dating" and let it take the course that it will. If everyone remains interested, it will just grow naturally.
The thing to remember is that poly dating is no different than regular dating. Don't forget to nurture the three pairwise relationships as being unique and special, apart from the triad as a three-way relationship. If you and your wife are the type to "court" someone in a formal way, then don't change your method now, it's worked so far right? By the same token, if the formality of courting is not something you did previously, don't start now.
It's obvious that she's interested. Frankly, this question would be best answered by sitting down the three of you and coming up with a plan that works for three.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker