How to store or copy the result of a formula field into another field

How to store or copy the result of a formula field into another field

Hi,

I want my formula field to lets say put the result of Single_Line1 + Single_Line2 into say Single_Line3

I tried a formula field of

Single_Line3 = (Single_Line1 + Single_Line2)

Which gets accepted but does not work.

What am i missing

Thanks