Incrementing within a Group/Subset

[Neilhwilliams]

I could do this even with my limited knowledge of SQL but I'm sure
there's a slick way that might be dead easy. My way hardly seems worth
the effort . If not thanks anyway

Col1 Col2 Col3
Q W
Q W
Q W
Q A
Q B
Q B
QX W
QX W
QX W
QX A
QX B
QX B
Every time there is a change of group (Col1 and Col2), I want to start
and increment Col3 so I end up with;
Col1 Col2 Col3
Q W 001
Q W 002
Q W 003
Q A 001
Q B 001
Q B 002
QX W 001
QX W 002
QX W 003
QX A 001
QX B 001
QX B 002

[Alex Kuznetsov]

Look up ROW_NUMBER()