Her reasons may be based on different things- 1/ feeling that she's missing onto things that she could have done/explored when it was just the two of you, 2/ feeling stagnant in the relationship 3/ having an early existential crisis, 4/ thinking that this behavior would somehow bring a sense of excitement in her life, etc. She herself ought to be able to explain this to you.
Now that the two of you have a family, there are things to consider if this is to take place.
Humans are not programmed to be monogamous. This may be a red flag that something is going on with the marriage and with her.
The best part about Just Answer is that you do not have to pay if you're unsatisfied.
Your question was "Why after the grief I was put through with the threesome answer I gave her while we were dating would she bring this question into our marriage now?"
The answers were speculations (not a psychological eval of your wife via this online Q and A service)
She is the best person to answer that question for you. Good Luck.