On the after update event of field1 write the code
If Me.Field1 = "100" Or Me.Field1 = "200" Or Me.Field1 = "300" then
me.Field2 = "2"
Else
me.Field2 = "3"
End if
But if it's a field that the user never change, write in the contril source
of the field2
=IIf([Field1] In ("100","200","300"),2,3)
Don't store this value in the table