I have to say Caprica, in many ways your 'poly' experience sounds similar to my own. Happily married, involved with a friend of both of ours.
We are not a formal triad by any stretch of the imagination, but it works so well (most days, unless me or my friend are feeling emotional then it gets dicey). We also are not in an 'out' poly relationship. We have not shared with anyone really.I agree just taking it day by , and following the natural progression is the best way to go. It will have it's up & downs but I think any relationship does.
So far we have had our situation for a year and a half with no major issues.
After 17 months of an emotional and physical relationship with my friend, I just started coming to terms with the whole 'poly' thing about 6 months ago. Until then I just tortured myself. I think if you stay honest, about what you and your spouse both want and let it progress it will be fine.
Good Luck to you!