any fla created in as3 file created in flash pro cs3 can be opened in flash pro cc.
you can open as2 cs3 created fla's in flash pro cc, but any actionscript will be lost upon saving from flash pro cc.
Flash CS3 file should open in Flash CC but the other way round will not work.
In general Flash Pro CS5 and above versions are future compatible and can open files either way. Some content may still get modified depending on what features it was created using.