Welcome to these forums. You seem to be well on the way to a successful triad, and coming here for advice from the wonderful veterans here is a great way to start. I'm glad you're asking these questions. It shows you've put a lot of consideration and thought into the relationship, so you already well ahead of many people who jump into these waters without much thought.
If there's any advice I can offer, it is to have the utmost respect for all parties, and talk, and talk, and talk about everything. And don't be afraid to speak up if you're ever uncomfortable about anything, and strive to create an atmosphere where your partners are equally at ease at letting their feelings be known.