I only have CS1 but it has the option to save as other AI versions such as v8. In the last 10 years I can never remember saving any illustrator files in any version number lower than v6. (Just checked, yes, Illustrator v6 was released in 1996.) v3 is so old Illustrator might not have been able to place bitmap or raster graphics.
In summary, save as v8 if CS3 allows you to.