Try something like this:

With
Values in every 3rd column
Beginning in C1 and ending in EV1

This formula automatically sums every 3rd cell beginning with C1
EW1: =SUMPRODUCT((MOD(COLUMN($C1:$EV1),3)=0)*$C1:$EV1)


Does that help?
***********
Regards,
Ron

XL2002, WinXP


"srinivasan" wrote:

I have a worksheet containing about 150 columns with data and in the last
column I sum up data based on requirement. ( totaling of every 3rd column
like C10+F10+I10+L10+O10+R10+U10+X10. When I insert one or two columns I want
that particular column included in the formula. Is it possible.? I am using
excel 2003