hi Filip,
with a multi-column report (using the multi-column option available in the Details section) you can't choose which groups are going to be multi-column or not...it's all or nothing with the Format Groups option.
based on your sketch though you can do this type of report with the following specs:
1) you have a main report which has one group on the AAA, BBB, CCC field
2) in group header 1b, you have a subreport which is linked on that field used in the group above
3) the subreport is formatted to use multi-columns including the second group
please see the attachment. extract the contents and change the .txt extension to .rpt. note that there's also a top n (n=2) in the group sort expert of the subreport to keep the number of group 2 columns to 2.
cheers,
jamie