Dave, Hi:
I cannot find any Expression Language function that will set a tag so this doesn't address your request to update three fields at once but it is an alternative to using Library Tools. After importing, choose a view that allows you to see the tracks for the target files of the album/albums that you wish to tag update. Then open the tag window and within the field you wish to update such as 'Band', enter the expression =[Artist] and press the ENTER key. This will propagate the [Artist] value, such as 'til tuesday, to the [Band] field for all of the selected tracks. Repeat the process for the [Album Artist] field. It is still 3 steps but the last two allow you to view the track and tag windows while you make the change. This is also the method I use when I want to copy one field to another. In essence, it is what you wish to do with the [Artist] field, copy the value to two other fields.
I've attached a sequence of images that changes a blank [Comment] field by using the expression =[artist] for the subject album tracks. The result is "'til tuesday" is placed in the [Comment] field for each track.
Someone with more experience may jump in to tell us that there is a way to do synchronous updating to fields. I'm just not sure what that is.
Cheers,
Lawrence