Help on Getting Wife interested in threesome MFM

Any advice for a very shy girl next door type that I know is wild in bed but says is happy with only me. She told me (before we were married while in college) that she had a fantasy about a threesome with two guys. She is NOT into anal but I've used a ten inch black doc johnmister dildo on her and she loves it half the time. We just had youngren and her sex drive is hit or miss. I really want open up her wild side. Thanks