are sensor from original compatible with new system

No part of the SS2 system will work with any part of the SS3 system, or vice versa -  SS3 uses encrypted signals and SS2 does not.

The only SS item which is system independent are the cameras (because they don't talk to the base system at all)

If you are interested, call SS and ask for any upgrade options, if your current system is pre-January 2018, they may offer 40% off a new SS3 system if you specifically ask for it.  SS3 does have some nicer features overall, worth the updgrade.