"The triple point of a substance is the pressure and temperature at which all three phases coexist in equilibrium."
Take water for example.
Let's say I put some water in a rigid container and add some heat.
Then both the temperature and pressure rise because the volume is constant.
If the triple point occurs during the heating, then that implies $V=\frac{nRT}{P}$, where $(T,P)$ is the triple point.
Does that mean the triple point can only exist in that size of a container??
Really interesting question, the answer is yes -- the triple point exists for closed thermodynamical systems. The key word here is being closed with some volume $V$ and non-interacting with the environment except for some stimuli. Every closed thermodynamical system has a natural thermodynamical equilibrium and each has an triple point unique to it inside the system.
Does that mean the triple point can only exist in that size of a container??
No. The ideal gas law you show applies only to gases. So the $n$ in that equation is not the total amount of substance, but only the amount in gas form.
Let's imagine you have two different containers (of different size), but you put the same quantity of water in. Then you seal it and move the temperature to that of the triple point.
Both containers will have the same pressure, but the smaller container will have more of the substance in solid or liquid phase, while the larger container will have more in the gas phase.
