Second it.I think 不过 is fine in written Chinese as well, although it's clearly less formal than the other two. I don't think there's a big difference between 可是 and 但是 either; I personally use them interchangeably...
I agree with you on the scale of formality. 不过 -> 可是 -> 但是不过 = spoken only
可是 = spoken/written - stronger than 不过
但是 = spoken/written - stronger than 可是
Is this correct?