One quick note about terminology: this will never be a "triad" because that basically means all three people are in a romantic relationship with the other two people. So that would mean both of your guys are romantic boyfriends to each other. Correct me if I'm wrong, but that's never going to happen.
In response to your question, Opening Up (Tristan Taormino) is a helpful book about non-monogamy. Many non-poly people have read it to understand what their poly partners or friends are feeling. Reading it yourself will help you relate to their concerns and give you tools to address them.
There are websites of course, but I personally think that books are better at putting everything in one place in a way that you're more likely to read it all. It's easy to hop & skip through websites, without getting to the heart of what you're looking at. The book might raise questions that they can explore further online, armed with more information.
Gralson: my husband. Auto: my girlfriend.
Zoffee: Auto's husband. Cue: Zoffee's boyfriend. Bookie: Cue's wife.
"A real relationship doesn't properly begin until the NRE burns away. That's when you have to start dealing with this person as an all-around human being, replete with irritating little habits. When disillusion sets in, love can begin."