As suggested by others you're never reaching the line that allows you to increment the counter. Try this algorithm that gives you all "xxx" combinations (000 to 999) and also avoids the generation of duplicates (Sayse's solution generates duplicates):
static void Combo(int a, int b, int c)
{
if (a < 10)
{
if (b < 10)
{
if (c < 10)
{
Console.WriteLine("( {0}, {1}, {2})", a, b, c);
Combo(a, b, ++c);
}
else
{
c = 0;
Combo(a, ++b, c);
}
}
else
{
c = 0; b = 0;
Combo(++a, b, c);
}
}
}