Question 2
Why did the motor car move when three people pushed it, having stayed put for one person and for two?
The right answer is the one marked above.
Because the resultant of the three forces was enough to move the car. The car and the obstruction stayed the same; all that changed was the size of the force applied.
Speed has nothing to do with it; the force does. The mass of the car does not change. And pushing in three different directions would make matters worse — forces add up only when applied the same way.