This is not necessarily advice, just experience.
I watched the first few times, and she also had some solo experiences with her encounter partner.
The very first time I was present - it wasn't a planned experience, but we had been working up to her being with another man for at least two years. It was still an overwhelming experience, even though I wanted it to happen.
It was simultaneously what I was expecting and hoping for, at totally different than what I had imagined.
If I had planned to participate in a threesome, I likely would not have been able to perform adequately the first few times. Reflecting on it, we probably had six to eight encounters, about half of which I was present for, watching, and the other half with her solo. Now I participate in about 75% - 80% of her encounters actively.