I bought the comma two about a month ago and really like it. Is there anything that comma three can do that the comma two can't do like lane changes without driver checking first, self navigation?
C3 only features in future includes navigation like you pointed out and possibly stopping at red lights and stop signs. They might try to get redlights/stop sings on c2 but its unclear if they would able either because of camera fov or processing power.
lane changes without user confirmation might be possible on certain cars with blind spot monitoring but definitely not on cars without it. For confident lane changes you not only need to know if there is car in blind spot, but also speed of oncoming cars. This isnt possible without side radars or cams. So even the navigation part when it comes out will require user confirmation