. set obs 5
. gen a=_n
. combin a 3
. list

     +--------------+
     | a1   a2   a3 |
     |--------------|
  1. |  1    2    3 |
  2. |  1    2    4 |
  3. |  1    2    5 |
  4. |  1    3    4 |
  5. |  1    3    5 |
     |--------------|
  6. |  1    4    5 |
  7. |  2    3    4 |
  8. |  2    3    5 |
  9. |  2    4    5 |
 10. |  3    4    5 |
     +--------------+