I want to see what my wife would think of having a 3some

Just find a younger, stronger, more well hung, and more handsome version of yourself and ask him to move in with you. Problem solved.

I'm assuming you're interested in an MFF?

Figure out if she finds women hot. Figure out her type. The various NSFW subreddits are great for that. I have found many women are attracted to a body type the reverse of what they are. My wife is curvy with a big ass and titties, and she is attracted to slim petite girls with small boobs.

Visit a strip club and get a VIP dance for the two of you. Have the stripper spend plenty of time grinding your wife, with her titties in her face and nibbling her ear and neck. See how that goes.

Assuming that goes well and she finds it hot, broach the subject. Many women will be interested in a 3-way but not in you fucking the other girl. If that's what it is, roll with it.