A B C
------
1 2 3
1 2 4
1 2 5
1 3 4
1 3 5
1 4 5
2 3 4
2 3 5
2 4 5
3 4 5

A = 1

WHILE A <= NUMBER_OF_UNITS DO
   B = A + 1
   WHILE B < MAX_NUMBER DO
      C = B + 1
      WHILE C <= MAX_NUMBER DO
         PRINT A B C
         C = C + 1
      END WHILE
      B = B + 1
   END WHILE
   A = A + 1
END WHILE

#include <stdio.h>
#include <stdlib.h>

int print_numbers(int number_of_units, int max_number)
{
   int a, b, c;

   if(number_of_units >= max_number)
      return -1;

   for(a = 1; a <= number_of_units; a++)
      for(b = a+1; b < max_number; b++)
         for(c = b+1; c <= max_number; c++)
            printf("%d %d %d\n", a, b, c);
   return 0;
}

int main(int argc, char *argv[])
{
   return print_numbers(3, 6);
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int print_numbers(int col, int nr, int max_col, int max_nr, char *line)
{
   char line2[BUFSIZ];
   char number[BUFSIZ];

   if(col <= max_col)
   {
      for(++col; nr <= max_nr; nr++)
      {
         sprintf(line2, "%s%d ", line, nr);
         print_numbers(col, nr+1, max_col, max_nr, line2);
      }
   }
   else
      printf("%s\n", line);

   return 0;
}

int main(int argc, char *argv[])
{
   char line[BUFSIZ] = {0};
   return print_numbers(1, 1, 4, 6, line);
}