data all ;
   set one(in=a)
       two(in=b)
       three(in=c);
   ;
   if a then seq=1 ;
   if b then seq=2 ;
   if c then seq=3
run;

proc sort data = all ;
  by seq ;
run;

data all (drop=seq);
  set all ;
  by seq ;
  if last.seq ;
run;