OK, due to being so-far anyway, the "bottom" of a V-shaped relationships I can only speak hypothetically, since I have not yet had that direct experience - that may well come, though, if our V becomes a quad.
I should also say that my preferred poly-style if definitely slanted towards the polyfi end, so this may not apply to others.
Once in the relationship, the assumption is that everyone involved has a functioning relationship - at the very least a friendship. This would mean that if the "third" saw an issue with the other two, it would be expected that they speak up, preferably to both of them, and voice those concerns, to either validate what they are seeing, or to have their mind put at ease that the conclusions they had drawn from the situation weren't reflecting the reality that the others were experiencing.
We have an agreement that everyone needs to be comfortable with any new person brought in. The assumption is that each person has the best intentions (a good-faith assumption) for the well-being of anyone involved. If someone saw red flags about any new person coming in, again it would be expected that they bring it up and discuss it openly, so that it can be addressed.
If things get rocky between any pair in the relationship, there is nearly always some sort of impact on the third - it's hard to avoid, really. Sometimes it's just feeling the empathy of what the other two are going through, sometimes it is being a little more flexible to give them room to get things sorted. It might even be a role of catalyst/facilitator to get the two to talk about stuff openly, if there is some breakdown in communication. That latter I could imagine could be a tad tricky, and would need to be handled carefully.
But like I said already, this is just our style and what we have agreed to - I won't pretend that it's the right way or anything, just our way. I know that there are others with far more practical experience than me, here - hopefully they can talk more to this from a real world sense than I can.
"Listen, or your tongue will make you deaf." - Native American Proverb