Moving sum by 12 months is easily calculated using PROC EXPAND. If you don't have PROC EXPAND in your SAS license, you can find workaround here by searching these forums.
As far as your weights are concerned
data weights;
set have;
weight1 = floor((month-1)/3)+1;
weight2 = lag(weight1);
weight3 = lag2(weight1);
run;
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