Hi Christoph,
Thanks for considering Syncfusion.
This is normal behavior of MS Excel also. If you set the first Group range to [1, 3, 1, 4] and second group range to [1, 5, 1, 8], the result will be single grouped range. Please refer the below modified code to get three grouped range in single row.
[C#]
//Group by row
sheet.Range[1, 3, 1, 4].Group(ExcelGroupBy.ByColumns );
sheet.Range[1, 6, 1, 8].Group(ExcelGroupBy.ByColumns);
sheet.Range[1, 10, 1, 15].Group( ExcelGroupBy.ByColumns);
Here is the video clip for MS Excel normal behavior
http://websamples.syncfusion.com/samples/XlsIO.Windows/P42551/main.htm Please let me know if you need any further assistance.
Best Regards,
Bhuvana