Question: Newton's third law states that the force object A exerts on object B is equal to the force object B exerts on object A (in the opposite direction). So if, for example, a 5 kg ball is dropped from a plane, the force of the ball is 50 (F = 5\*10), so if the ball is exerting a force of 50N on the air molecules around it (even if it is one air molecule), shouldn't those molecules exert 50N on the ball too? And so shouldn't that ball always be in terminal velocity?

Answer: 
There are many cases that you can use statistical mechanics for various reasons. Regarding your question about the ensemble, it is possible to use them even in a single system.

Let's consider the most familiar example, the ideal gas, in which various physical properties have be derived. Now, let's divide the system with volume $V$ into $N$ sub-system with volume $V/N$. The $N$ should not be too large such that $V/N$ can be considered as microscopically large. Hence, all of the sub-system have the similar (mean) macroscopic properties as the originally system, but now you have $N$ realization of an ensemble instead of a single system! Even for interacting system, it is usually possible to divide them into multiple sub-system, if the division is larger than correlation length. Each sub-system can now be considered as a realization of the ensemble, and statistics can be performed for this ensemble. 

On the other hand, if the system has the [ergodic](https://en.wikipedia.org/wiki/Ergodicity) property. Then the system at different time may be considered as a realization of the ensemble, with sufficiently long time separated. The time averaged properties will be the same as the ensemble averaged. As an example, the temperature may fluctuate over very short time period, but a slow response thermometer can report the average temperature.
We can use statistical mechanics whenever the phase space distribution of the system is independent of time. This means that the distribution has to be a function of the constants of motion of the system which are $E, \vec L, \vec p$. Generally we have control over $E$ and not the others so we generally deal with systems whose phase space distribution is purely a function if $E$. 

Now the way we go about applying this is by first working out the allowed energy level for a single particle. Here we consider the total Hamiltonian (potential) that a particle sees and then find out the allowed energy states. Note that this is not the same as taking a system of particles and choosing one of them and treating rest as heat bath. No no. We are solving for possible energy levels for a single particle. Once we have that, we will fill them up with the total number of particles such that the total energy is equal to the desired value. 

And there could be many ways of filling it such that we get the right total energy. These are all the possible microstates. This method works when the interaction between particles is negligible, because we ignore interactions when we solve for single particle states.