Trust is a two way street. It is not merely doing what you say you will do which is where I see Elemental needing to put work in. It is also saying what you mean as BP (and her hubbo I hope too) discovered some work needs to be put in.
As well it is about believing you can say what you mean and not be "punished" or have it fall on deaf ears.
When Elemental and the GF discovered they wanted some alone time, it should have been shared first with BP. When BP was struggling to feel close to them as a result of being out of their loop, she should have been perhaps clearer about the emotions she was feeling over it and not push herself too much too soon to get over it. I commend the GF for telling Elemental he had to spill about showing up at the bar or she would tell BP herself. It means she knew what it could mean to BP and cared about that more than she cared about what was building between her and Elemental.
Elemental, is there any insight you might have to what gave you the impulse to go around BP in a more vee pattern than the triad situation already afforded you? Did you feel challenged by their rapport or made to feel like you would be eventually hedged out yourself? Do you struggle to not be the center of attention in group situations? Or did you and the GF really want to do away with the rule of all involved but felt BP was not receptive to the idea? Did some part of you know it would create a split between them and make you more the focus of them both?