Copy a formula while changing column references

Hi,

i have a really simple question about copying a formula, but I didn't find any information about that in the help or on the internet.
So the problem is that I want to copy a formula down a couple of rows and make the reference to columns in the formula change (not the rows). Sounds little confusing but here an example:

Sheet1:
_A_B_C ...
1 0 7 7 ...
2 7 0 7 ...
3 7 7 0 ...
... ... ... ...

Sheet 2:
_________A_________
1 SUM('Sheet1'!A1:A3)
2 SUM('Sheet1'!B1:B3)
3 SUM('Sheet1'!C1:C3)
... .............................

So the question is how can I copy a formula down in Sheet2 to achieve that just the column is changing and not the row number. To avoid that the row number is changing I can use a $, but still I don't know how to make the column change. Sounds pretty easy but I can't manage to do it or find some information about it. Anybody here knows a solution for this problem?

Regards,

sauron