If I understood correctly, the requirement is to set column width based on the number of groups.
If this is correct, you can set Binding rules
. One for the group column to control it's visibility (as you mentioned, if there is only one value for it), and one for the rest two columns to control their widths (to fulfill the page).
If you need further help, please send us the report definition along with some sample data to investigate further and provide more specific suggestions.
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