YIKES!!! Not cool of J to say that he doesn't care about Bs feelings. I'm in a triad and I don't have much experience with a V relationship, but I'm pretty sure that feelings like those aren't going to make for a healthy relationship. For anyone.
You were right on the money to put J in his place, but was that enough? Does J think that eventually you'll leave B to be with him? Unfortunately when someone makes a statement like "I don't care..." that leads me to believe that maybe they're not really poly at all. I don't know enough about them to really say that, but just an observation.
Keep talking to both of them about what's going on. Fact is that it affects EVERYONE and EVERYONE needs to be in the loop, regardless of whether they want to hear about it or not. This is brand new and like any new relationship needs a lot of attention at the start.
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!