data data1;
  subject = 1001; output;
  subject = 1002; output;
  subject = 1003; output;
run;

data data2;
  subject = 1001; output;
  subject = 1002; output;
  subject = 1004; output;
run;

data data3;
  subject = 1001; output;
  subject = 1004; output;
  subject = 1005; output;
run;

data out;
  merge data1 (in=a) data2 (in=b) data3 (in=c);
  by subject;
  source = 100*a + 10*b + c;
  put subject= source= z3.;
run;