Actually I don't think they are convex, but are telescopic and work more like a telephoto lense, so each mirror has a different position of the lense.
(IE)
Incab rearview mirror is normal (or further away)
Drivers mirror is closer (or further away)
Passenger mirror is closer again (or further away)
Like most things, it's something you need to adapt too or change the glass, as mentioned above by DC.
Baz.

