I can tell you that while pregnant or immediately after (ie 6 months) is NOT the time to imply you are not interested in your wife.
While you can say that is not the case, bringing up adding a 3rd she has already said no will likely make her feel that way.
My suggestion.....if you love your wife you need to cut the relationship with A back to work only. Don't allow yourself to go there until your wife is ready. And be prepared she may still say no.
If she continues to expect monogamy what will do you?