I constantly feel like a third wheel

I used to be friends with two people who recently began dating, and our friendship has changed since that happened. We used to do everything together but, since they got into a relationship, I feel excluded from a lot of things. I don’t resent them for wanting privacy or doing things on their own, but I feel sad that they can’t accept an old friend when it comes to social activities. I feel like the odd person out whenever we meet, as if they have access to secrets that aren’t revealed to me. It sometimes feels as if they would prefer it if I didn’t meet them because that would make it easier for them to manage. Should I assume our friendship is over?
Why assume anything without making an attempt to address this? It’s normal and understandable for your friendship to change, given that they are in a different kind of relationship. It’s also normal to feel like the odd one out because people in a relationship have a different mode of private communication that other people are excluded from. It doesn’t mean you have to write this off though, because that will adversely affect all three of you. Why not take some time off from this friendship, and allow them to get used to the idea of being together? Tell them why you feel this way, and why you want to give them privacy until they are open to the possibility of rekindling this friendship on new terms. If you accept that the nature of this friendship has changed, and that this change is normal, you will all be able to find a way forward without losing what you once had. It will just be a different kind of friendship for a while, until you all find the tools to navigate this.

Is it normal for my girlfriend to want to meet an online friend?
Why isn’t it normal? It’s only by meeting someone in real life that she can gauge whether this friendship is real.