Welcome to our forum.
You are certainly right that it's possible to love more than one person at a time, and even if it's a little scary in the beginning, it will get easier.
It concerns me of course that this other woman's husband doesn't know what's going on, even if she's not happy with him. You need some kind of exit strategy for that part of the situation. If nothing else, she needs to get a divorce. Or get into marital counseling or something, but this can't be kept a secret from her husband forever. Hopefully it can be resolved sooner rather than later.
You have a good thing going with this triad relationship with this woman, and it is worth hanging onto. If you'll look around at our different threads here you'll see that there's so much to learn, and you can always post your thoughts and questions.
I'm glad you could join us.
Love means never having to say, "Put down that meat cleaver!"