While spending time with her might mean spending less time with you (although he could spend time with both of you at once, or the time he spends with her could come out of time he used to spend alone), love doesn't work in the same way.
If you think about friends, for instance, having a new friends doesn't mean you like the existing ones any less. Actually, if they get along it strengthens your bond even more! Poly is similar, being in love with her doesn't make him love you less. Knowing you like her and are willing to let him be free and happy will make him love you more, and love for one person tend to make you love others more.
Think about it, if you watch a romantic movie, doesn't it make you love everyone you love more? If you see something you find cute (a child, a kitten, a puppy) doesn't it make you want to hug the people you love more? Affection creates more affection, the more loved you feel, the more love you have to give. It's not like a piece of cake. Giving her part of the cake doesn't mean you get less, you know what I mean?
I don't usually like talking about children, because people are quick to say "I don't have sex with my children" but since we're not talking about sex at all here... A mother who has a new child isn't going to then spread her love between her two children. Often she'll find that she loves the first one more as a result of loving the second one. Do you see what I mean?
Relationships evolve on their own, and there is no way to know if your relationship will end someday, but I'm fairly confident it's not going to be because she's "replacing" you. That's more of a mono kind of thing: if you can only love one person at a time, then to love a new person you have to leave the previous one. When you are poly, no such rule apply. If he left you, he wouldn't gain anything from it, he would lose you. Why leave you when he doesn't have to choose and can have you both? That would mean his choice isn't between being with you or being with her, but being with you or not being with you.
Do you see what I mean here? Since he can be with her either way, being with you is something completely independent here.
It's normal for you to want to feel included, I believe. It's definitely what really matters to me, at least. And knowing they went behind your back, that's probably the worst of it, because that means they didn't trust you, they didn't include you in it. But if you're included in it, then you're also part of that relationship, You can be friends with her, and then leaving you would mean that they would both lose you. It doesn't have to be you against her, especially since you seem to be getting along.
You might want to dry going to dates with both of you with him. Of course it depends of your personal comfort, but it might help you to see them together and see that he's not pushing you aside to be with her.
I'm a bit rambling I guess, but I hope this way helpful to you. Good luck!