Do you mean you just want to know what weights to use? You can just use 0.17, 0.33 and 0.50 for year 1, year 2 and year 3, respectively.
data for_model;
set for_model;
if base_year=1 then wt=0.17;
else if base_year=2 then wt=0.33;
else if base_year=3 then wt=0.50;
run;
proc reg /* or whatever model proc you are using */ data=for_model;
WEIGHT wt;
** other model statements ;
run;
There's nothing magic about the 0.17, 0.33 and 0.50. You could just as well use 17, 33 and 50, or 34, 66 and 100 -- they will also produce the same result as long as the weights are of the same relative magnitude. Also note that if you simply replicated observations in your input dataset (i.e., make 17 copies of each base year 1 record, 33 of each base year 2 and 50 of each base year 3) and then left off the weight statement, the same thing will happen in terms of the point estimates (NOT that you should do that, as it will affect standard errors / CIs).
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