Communicate, communicate, communicate Jealousy, envy, scheduling problems, can all be minimized by effective communication. We've gotten into trouble when we've assumed our spouse or our sweetie thought like we did or had the same motivation that we did.
Be sensitive to the primary/secondary difference A couple like us who have been together for many years cannot help be closer to one another than a third person. Anyone coming newly into our lives will necessarily start out in a secondary relationship. Over time we would hope to develop a balanced tri-primary relationship. But to begin with, it won't be and can't be a fully balanced relationship. Be sensitive to the new person's needs not to be shut out by your own intimacy.
Avoid secrets If you share thoughts and feelings among the couple that you keep from the third person, you're building a barrier to intimacy.
Respect the risk the single person is taking If the experiment doesn't work, the couple still has intimacy, security, and love in a socially accepted context. A single person loses all these things. So they may have more at stake, and be more risk averse.
Encourage one-on-one time among each dyad in the triad A successful three-way relationship incorporates three successful two-way relationships. Allow time for, and nurture all three of them. Include sexual, conversational, out-in-the-world, and family activities in these pairwise periods.
Adjust to changes in the couples-bond If the triadic relationship is succeeding, the couples bond will be balanced by ever-strengthening bonds with the once-single partner. This will cause the couple relationship to change. Just because that's your goal doesn't mean it will always be comfortable.
Take it slowly Three people won't fall in love, take emotional risks, and alter life plans at the same rate. Respect everyone's discomfort levels with the changes you're going through, and take it no faster than the slowest of the three is comfortable with.