The SS2 tools in SS3 will work but it would be difficult to ignore SS3 workflows in SS3 simply because the SS2 roadway designer and site modeler commands are not available and have been replaced by new OpenRoads corridor modelling and site design tools. SS3 is a released product and as such Bentley will provide the usual support for production work.
I'd suggest to install SS3 side by side with SS2 until you are comfortable with the new workflows before turning off SS2. This is primarily to accomodate your learning curve in the transition to SS3.
Regards
Paul