There are a few models that I've observed in relationship dynamics regarding support of one's partner(s). One idea is that, as each of us are responsible for our own emotions, the support that a partner provides comes from the sidelines. Let's say that A is struggling with feeling jealous/insecure over a new connection with C that B is forming. B would listen to A talk about what that was like, then encourage A to get past that, without changing anything about their own behaviors, or considering that they might share a responsibility to help their partner through that tough spot. Your issues, your emotions, you deal with it. Passive support.
Another idea that is my personal preferred setting is that, in the same circumstances, B would not only listen to A, but they would mutually form a plan, perhaps even including C, to help A work through those uncomfortable feelings. This wouldn't be done by avoiding triggering A by cutting off contact with C, but by providing a higher level of support or additional reassurance to A, making the budding relationship with C feel more safe and positive. Active support.
The third option that I've seen utilized is that when A feels uncomfortable with B's connection with C, B is expected to sever that connection, so that only the most nonthreatening relationships are allowed to continue. This is pretty classic avoidance, and based on the idea that the original couple is paramount, and anything/one that pushes up against that safety bubble should be ejected. This is most often seen early on in people's poly experience, or following a very poor experience, when there is still significant healing left to be done. Avoiding support.
Let's get into scenario #2, since it's my favorite! An example from my current life is that S and I are both involved with JA, but, as I've been somewhat involved with the transition in my relationship with PG for the past several months, they feel further advanced in the relationship emotionally than she and I do. They've had more time together, more intimacy, and are happily romping through some significant NRE! That's been bumping up against my fears about being left or replaced, or that maybe S will find himself gravitating towards emotional serial monogamy, and so we've been looking at those concerns together.
Fortunately for me/us, JA is very interested in working on a growing sustainable connection with each of us, as well as a three person dynamic that has a stable foundation. Also wonderful, S is cognizant of my current emotional challenges, and wants to invest in making sure that I feel valued and important in his life, even in the throes of his NRE. For my part, I don't want to be a limiting factor on their connection, and my focus isn't on feeling good about everything all the time, nor am I interested in martyring myself because I "should" be fine with whatever choices my partners desire.
So how does that look? It looks like a lot of conscious, vulnerable, honest communication. It looks like sharing when something feels unsafe to me, and asking for additional reassurance or clarification on what's happening. It looks like showing my places of weakness (and I really hate to feel weak!), and bringing up having feelings that are related primarily to past experiences and other people. It looks like continuing to grow the relationship that JA and I share, and being aware that she has her own challenges entering such a strong dyad connection during a period of great change. We are all wearing multiple hats with each other: as a partner, a metamour, and as part of a triad. It is a VERY active process of support for each of us. This wouldn't be workable if everyone wasn't putting into the pot, and providing for each other.
Relationships aren't "sink or swim" structures. The times where I've seen people rise above their own past experiences, they've put a huge amount of effort into that process, and one of the most helpful ways to accelerate that process is by having friends, partners and/or family that do more than say, "Sorry you're having a hard time/feeling that way." and wait for their loved one to figure things out on their own. When someone is struggling, toss them a float, hold them above the water. Show them how to swim, and celebrate when they cross the English Channel of emotional challenges, or even just make it over the puddle that looks really big.