If you normally hang out as a foursome, then I think it was a bit shit not to invite her.
There’s nothing wrong with saying “me, Jo, and Sarah want to try the menu at XX restaurant so we’re thinking of going on XX date. Would love you to come but I know you said it was over budget for you last time. Just wanted to give you the option of coming too as I’d never want you to feel left out - totally understand if you can’t stretch to it at the moment.”
For context, I have autistic DC, one with high needs. I can’t do a lot of things that my friends do. I go to things as much as I can but anything overnight or a distance away is out of the question, sadly.
My friends pretty much always include me in the invite and I appreciate it so much. It’s hard when friends are doing things that you’d normally be part of, but knowing that you were thought of and wanted makes all the difference.
One friend had a big birthday last year and organised a week’s trip to an overseas destination. Friend forgot to tell me. I found out just before they flew. Knowing they’d all been chatting and planning it for a year but no one had mentioned it did really hurt, tbh.
If you tell her and she gets shitty, that’s another matter. You’re totally allowed to organise things she can’t join in with. It’s the secrecy that’s so hurtful.
If you don’t usually hang out together then it’s not really an issue. But if you three don’t usually meet up without her, I think you should have said something.