Can you put a reluctor ring on a 3 series axle or is the reluctor ring made onto the 4 series axles?

 

Are you asking about the 3-channel reluctor that mounts to the differential carrier? If so, what "4 series" carrier are you using? Or are you asking about the 4-channel reluctor gears that fit on the axles, behind the hubs? I'm not aware of any part of the 4th Gen rear axle assembly that is "4-series". You can get 4-series carriers in a 12-bolt though, if that is what you are using.

 

The 4 channel cars (speed sensors at the axles) have the reluctor made into the axle. Axles, themselves, are not series specific.