Its not quite that simple. While with 3 cars Mercedes may have bagged many more points per round so far, the amount they would have to attain to be beyond catching also increases as the points available is higher.
With 3 cars scoring the maximum from a weekend would be 58 rather than 43, so the lead needed would be 58 x number of races left. So at the moment with 6 (5) races left Mercedes would need a 348 point lead in a 3 car scoring formula, compared to the 258 they currently need to be unbeatable at this stage with 2 cars scoring.
/Man maths. Lunch time.
It would be mathematically possible but not realistically.
It's too much 'tampering'
3rd driver used as a roadblock if no points. Still can push others down grid
I dunno. Maybe it would be better to weed out the back markers