I sometimes worry not that what we're doing is immoral but about how other people, usually strangers, would react if they knew. Human beings do terrible things when they feel their worldview is threatened.
In the case of us three, would it have been more moral to cast aside a really good friend in the name of coupledom because that's the moral norm? Doesn't seem like it especially when that decision would have cast one or more of us into crippling depression, anxiety and near poverty. Do I still worry that this wasn't the best decision for E and that he might be happier in a more traditional relationship with only one other person? All the time, but he assures T and I that he's more than happy with our arrangement. T says the same about his feeling about our situation.
Can polyamory be immoral? Hell yes. If you're hiding being the banner of polyamory as a way to duck the responsibility of your primary relationship or act in ways that will knowingly harm your more steady relationships, that not exactly moral either.
I like Dan Savage's rule about coming out to loved ones - give them a year to go through all the emotions and adjust. If they are on board after a year, fantastic. If not, cut 'em loose.