Drag and drop SUM value

Hi all,

Maybe I need another coffee cause the answer is simple, but maybe it isn't. Either way here is my dilemma:

Row Column
1 A
1 B
1 C
1 D
1 E
1 F
1 G
1 H
1 I
1 J
1 K
1 L

Assume the above have numerical data in them on SHEET 1. I want to be able to use the SUM function and add up ABC, DEF, GHI and JKL in Row 1 Columns A (ABC) B (DEF) C (GHI) and D (JKL). Right now I use the =SUM(Sheet1!A1:C1) to get the value of that put into cell A1 on SHEET 2. The problem is that if I use the drag handle from that cell it will return =SUM(Sheet1!B1:D1) in cell B1 on SHEET 2, but I want it to return =SUM(Sheet1!D1:F1). How to get this?

You can use OFFSET for that:

=SUM(OFFSET(Sheet1!$A$2,0,(COLUMNS($A$1:A$1)-1)*3,1,3))
and fill across.