It truly does take an enormous amount of empathy to pull off a triad. Of course people are going to be full of NRE. I guess one thing that helped us was that our girlfriend was more into women than men before joining us. This meant that I didn't start out feeling like I was somehow 'letting her sleep with my husband', because the relationship she and I had together was very natural. I think the place where many triads fail is the relationship between the same-sex partners. If that relationship is not strong you just end up in a tug of war over the heterosexual relationship.
Remember, there is NRE in each relationship, including the three bond. I suggest sitting down and discussing where each of you sees the relationship going. It sounds weird and awkward to do such a thing but it helps. Kind of like how you have to sit down and discuss condoms/birth control. Both my partners are more introverted than I am, so it was always up to me to start these conversations. Like it or not, that is one of my roles.
Being pro-active definitely helps. I'm not sure what you mean by 'spontaneous' but in our world communicating and making sure people are on the same page actually make spontaneity easier in the long run.