Maybe I'm overlooking the obvious...but I'm having a hard time grouping columns.

Here's the goal:

Columns A - L are the 12 months... Jan-Dec.
I want to group 3 columns (3 months) at a time. So I'd have 4 column groups side by side.
I select the entire first 3 columns...data...group/outline...and group...creates the group fine.
I then select the next 3 columns (next 3 months)...to be in their own group. When I try to group them...it just ads them to the first group...making one large group.
The first 3 columns are not selected when I do this.
I've also tried playing with the summary details in the group settings.

Thanks!