Nailing your core beliefs in 3 sessions would be pretty A-mazing!!
All our behaviours, everything we do, everything we react to, gets traced back to how we were feeling. That's a lotta stuff. Feelings are slippery little buggers - they can be all over the place as well. And they all get traced back to the (sometimes microsecond or even unconscious) thoughts whizzing through our head 24/7. Between the behaviour mess, the feelings mess, and the thoughts mess, that's a lot of stuff that cbt is teaching us to process "in the moment".
And all of that great big mess comes back to our core beliefs. And we've got a whole stack of them. Some good, some meh, some not so good. And under all the mess in our behaviour, our feelings, and our thoughts, it's shifting the core beliefs that's gonna change our world. You gotta know them like you know the way you make your coffee in the morning.
So 3 sessions on core beliefs? I reckon it could be done. But you'd have to know yourself like, on a totally Buddhist kinda level, to nail it that quick.
Maybe she's just got nothing else to offer. Or maybe this stuff, like all therapy, takes more time than we want it to;)