Well, being the hinge is stressful. Sometimes it feels like your partners expect you to take care of all of their feelings for them, and it's overwhelming.
On the other hand, it's good to talk about things. If you keep it hidden, it can't be addressed.
I would suggest bringing things up in the least aggressive, most constructive way you can, and suggesting solutions when you bring up a problem. For instance, with a random example: "I feel neglected when you go out and I stay at home alone. Since I have that class on Tuesday evenings, do you think you could plan more dates at that time rather than on Mondays?"
Now, this is completely off the top of my head, but in this example, it makes it clear it's not her fault that you feel neglected, but that the feeling neglected is still something you want to be addressed, and some re-scheduling could fix it.
I think it's important for her though that she doesn't expect everything to go through her, and at the same time expect it not to be stressful. Either you deal with some things without her, or you bring them up with her and it will be stressful. It's her choice. She seems to want to be in control, which is something I can relate to, but she'll have to learn to let go of some of that control if she doesn't want to burn out. You don't expect her to singlehandedly fix everybody's problems.
As for saying her position is the worst, she needs to realise that while being the hinge has its own set of difficulties, you two have to deal with a lot of things she doesn't. Saying you're not cut out for poly because you sometimes feel jealousy or insecurities would be like saying she's not cut out for it because she feels stress. It's important that all three of you know and realise that you all have your own challenges, and you need to support each other through them.
In short, I think she can't complain about all the stress, but simultaneously insist on taking care of everything. She's only human, there are three people in the relationship (although it seems there is more dating as well? She might want to slow down and focus on existing relationships before she adds more. There is a point at which it becomes unmanageable after all), she can't expect to deal with everything, you need to work at distributing responsibilities, and at solving problems together.
How much time do you spend with him without her present? Do you pretty much only have contact when she is involved, or are you friends of sorts at this point?