I don't think you can ask her why she cropped the photo. There was nothing wrong with that - she feels closer to the others, and wanted that photo.
What was wrong was sending it to you. If you honestly think she did that deliberately, then you need to fade away from her, as that's unkind. If it was an accident, then you have to decide what you want. They don't see you as close a friend as they see each others, and that's just how it is - not something for them to apologise for. If you still want to be friends with them, you may have to accept that. Or if you don't, it's fine to move on. And it's fine to be hurt by it, as well, even while accepting that there's nothing wrong with them being a closer three with each other. You already know that they aren't as close to you. Hurtful as that may be to you, that in itself is not anything for them to be guilty or apologetic for.
If you do want to remain friends, on the same terms you are now - not as close to them as they are to each others - they I wouldn't say anything. If you want to move on, go for it.
I wouldn't use a confrontation like that to judge whether to continue the friendship or not. What sort of answer could she give? They won't want to admit they are closer to each other, but it's the truth. There's nothing they could say, or apologise for, that would change that. Asking why she sent it to you is different; if you really want to know, then you can bring it up - but I doubt the friendship would survive that, so I'd be sure that's what you want first. They are likely to get defensive and to make excuses, as that's what happens in confrontations, and then you'll still end up feeling unsure if it was deliberate or not. What would you like them to say? You can tell them you were hurt - but what can they do that would change anything? Pretend it's not true, that you're all equally close? I don't think you will be able to make any decisions based on their reactions tonight.
I think it comes down to you deciding whether you want to stay friends with people who are closer to each other, or not. There's no right answer to that. Some people might find it acceptable, and others not. It doesn't mean that there's anything wrong with you. I am friends with other people who are closer to each other. It does hurt, absolutely. But there's not much I can do about it, either. They are doing nothing wrong. And it's up to me whether I want that level of friendship, or whether I want to try to find my own separate crowd of 'best friends' that I'm more equal with.
Hope you find a solution you are happy with.