Would all these three things have the exact same effect on the flight duration of a glider?
Scenarios 2 and 3 would be almost exactly the same. Scenario 1 would be very different. However, the glider would be able to stay up for almost exactly the same amount of time in all three scenarios.

The basic summary of what would happen: In scenario 1, both the weight and the mass would be doubled. In scenarios 2 and 3, the weight would be doubled but the mass would remain the same. 

The simplest way to distinguish scenario 1 from the others is to pitch into a vertical climb (or a vertical dive). In scenario 1, the glider will accelerate downwards at $1\ g$, but in scenarios 2 and 3, it'll accelerate downwards at $2\ g$.

Scenarios 2 and 3, on the other hand, are almost completely identical. Specifically:

Scenario 2 simply adds more gravity. Pretty straightforward. 

Scenario 3 consists of having the ground accelerate "up" at $1\ g$. We can look at this scenario in a frame of reference where the ground remains stationary. In this frame of reference, everything is just like the real world, except that everything is now experiencing an inertial force equal to $1\ g$ straight down.

How does an inertial force work? Inertial forces feel just like gravity. In fact, the only differences between the extra forces in the two scenarios are:


In scenario 2, the direction of the extra force changes as you move horizontally, because the direction of "down" changes as you move. In scenario 3, the extra force is in the same direction everywhere: namely, the direction opposite the direction of acceleration. 
In scenario 2, the extra force gets weaker as you get farther away from the earth. In scenario 3, the extra force is equally strong everywhere.  


Both of these differences are probably too small to measure. 

Now, finally, I said that the glider will be able to stay up for the same amount of time in all three scenarios. This is because, although the mass of the glider differs between the three scenarios, its weight is the same in all three scenarios. And if the glider is flying at a constant speed in a straight line, weight is the only quantity that matters; mass is now irrelevant, because the glider is not accelerating (in the frame of reference where the earth is stationary).