And you found your way here!
A number of people can offer advice on creating a healthy, positive relationship with a third person.
It's good to be honest with the other person from the first. Sure, anyone who needs an exclusive, monogamous relationship will shy away. But leading them on by pretending to offer some form of monogamy and only later revealing that you're looking for a poly relationship will absolutely cause problems.
It's also good to think about what form your relationships might take. Often what you describe is a vee, at least at first, with one person -- you, the hinge -- having two partners. My feeling, and I think others agree, is that it's by far the best if the two people on the legs of the vee can be friends. Certainly they should know about and have met each other.
And also think about how to make sure everyone gets at least some of their needs met. There's that touchy time when love is new -- new relationship energy, NRE -- when it might be easy to forget to give attention to your fiancee. So some initial guidelines might be appropriate, maybe an agreement to have one date night a week with new friends, and a couple of special evenings for your fiancee as well.
But consciousness and awareness is really key -- whether in honesty, relationship shapes, or time management.