☭ cat /tmp/s.c
#include <stdio.h>

int main()
{
    const char *t[] = {
        "22:33:44.555",
           "33:44.555",
              "44.555",
    };
    size_t i;

#define SEP "%*1[.:]"

    for (i = 0; i < sizeof(t)/sizeof(*t); i++) {
        int a = 0, b = 0, c = 0, d = 0;
        int n = sscanf(t[i], "%d"SEP"%d"SEP"%d"SEP"%d", &a, &b, &c, &d);

        switch (n) {
        case 4:  printf("h=%d m=%d s=%d u=%d\n", a, b, c, d); break;
        case 3:  printf("     m=%d s=%d u=%d\n", a, b, c   ); break;
        case 2:  printf("          s=%d u=%d\n", a, b      ); break;
        default: printf("fail: n=%d\n", n);                   break;
        }
    }

    return 0;
}

☭ gcc -Wall -Wextra /tmp/s.c && ./a.out
h=22 m=33 s=44 u=555
     m=33 s=44 u=555
          s=44 u=555
☭