Hello and welcome to the forums!
For the regular readers of the forum this type of situation is not at all uncommon - you are not alone!
Many people seem to start their poly adventure with the goal of an equilateral "triad" type relationship - often this takes the form of an existing couple that "adds a third" person to the relationship. Issues then come up when one or the other of the relationships doesn't progress as easily or as quickly as the others, which can trigger jealousy and insecurity, and it is at this point that a lot of people find their way here.
Poly relationships are complex! There are many relationships involved - you might want to read this article
on polymath and the exponential complexity of adding more relationships.
Galagirl has pointed to some links that may be helpful and there are a lot of articles and threads about dealing with jealousy - so there are a lot of resources out there.
I would point out that it is entirely natural for different relationships to grow and change and wax and wane at different times and in different ways. Expecting two different relationships (yours+girlfriend and hubs+girlfriend) to be at the same place at the same time is not really realistic and can lead to resentment. Each relationship in the polyship needs its own care and tending. So, rather than focusing on what THEY have, it can be more fruitful to focus on nurturing your relationships with each of them (and yourself).
While it is perfectly OK for you to ask for what you need (time alone with each of them, time together, for them to tone down their interactions in front of you, whatever) it can be very difficult for people who are caught up in their own NRE (new relationship energy) to really SEE how what they are doing is affecting other people. But communication is key, they can not help you get what you need out of your relationships if you aren't telling them what you need.
It is not unusual for Triad relationships to take on other forms over time - this doesn't always mean that the Triad breaks up completely but it is not unusual for these to transition to a Vee configuration. (Numina's blog
is one person's journey after the transition - you can look up her earlier posts for the backstory).
Your fear that they may decide that they just want to be with each other is a common one...and, ultimately, not something that you have control over. But the beauty of poly, for a lot of us, is that people DON'T need to choose between loves. So you nurture your relationship with your husband, you work on your relationship with her - he has to be willing to nurture his relationship with you, she has to be willing to work on her relationship with you. Their relationship with each other doesn't need to dictate your relationship with each of them separately. Ultimately, some relationships continue and others don't - that is the nature of relationships.
(I'll dig around and see if I can find some recent threads on similar situations for you.)