how to insert one up numbering based on another field?

Here is my scenario:
table 1

field1 field2 field3
abc 2
abc 100
abc 100
def 1
def 100
ghi 100
ghi 100

Everytime if the code finds '100' i want field 3 to start with number 1. so the expected results i'm hoping for
field1 field2 field3
abc 2
abc 100 1
abc 100 2
def 1
def 100 1
ghi 100 1
ghi 100 2

how do i do this output by applying vbcode and i belive do with a nested if? I'm pretty new with access and vb coding so trying to find out how it can be done? any help would be appreciated.

Thanks

re: how to insert one up numbering based on another field?

is this in Excel or Access?