Asking if she has anything to tell you is one way, though that's a leading question that telegraphs suspicion, which may in turn put your wife on the defensive. That's not the most comfortable place to be when revealing intimate details of one's life. Instead, you may want to nudge her by saying something like, "You have a lot of fun with [your third's name], huh?" Or, "You really like her, huh?"
The Guardian's Saturday magazine is looking for throuples to talk honestly about the experience of love and commitment. We're particularly interested in talking to throuples living together under one roof, as well as throuples who are raising children as a unit of three parents. Is it easier to manage childcare duties when there are more adults in the room? Or more difficult? It is important that all parties agree to being featured.