How can I use the Forecast function on non-adjacent columns to avoid having
to use the following equation.

=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)))