Forecast Function
 

How can I use the forecast function on columns that are not adjacent to each
other, or alternately, is there another function I can use for this.
Otherwise, I have to type in the whole regression line equation thusly,

=AVERAGE(L3,AB3,AR3)+((((3*((L3*1998)+(AB3*2001)+( AR3*2002)))-((L3+AB3+AR3)*(1998+2001+2002))))/((3*((1998^2)+(2001^2)+(2002^2)))-((1998+2001+2002)^2))*(2003-AVERAGE(1998,2001,2002)))

I'm forecasting 2003 data from 1998,2001, and 2002 data.

Thanks, X