data one;
input indcd $ year x;
cards;
a 2008 1
a 2009 2
a 2011 4
a 2010 3
a 2012 5
b 2010 3
b 2009 2
b 2011 4
b 2012 5
;;;
run;
data two;
set one;
by indcd;
if first.indcd then count=0;
count+1;
sum=x+lag(x)+lag2(x);
if count>=3 then sum=sum;
else sum=.;
mean=sum/3;
drop count;
run;