Gibbs Phase Rule Calculator
Water's triple point has zero degrees of freedom, which is what made it a definition of the kelvin.
The formula
F = C - P + 2
What the rule counts
F is the number of intensive variables — temperature, pressure, compositions — you may set independently before the state of the system is fully determined. C is the number of chemically independent components and P the number of phases in equilibrium. The 2 is temperature and pressure.
The triple point is forced
Pure water has one component. Put ice, liquid and vapour in equilibrium and P is three, so F is zero: there is exactly one temperature and one pressure at which this can happen, and neither can be adjusted. That is what makes a triple point a reproducible fixed point, and it is why the kelvin was defined by water's triple point from 1954 until the SI redefinition of 2019.
A negative answer means it cannot happen
Four phases of a one-component system would give F equal to minus one, which is not a state of affairs but a contradiction. The rule is therefore also a test: if it returns a negative number, the arrangement you described does not exist.
Where these models stop
Michaelis-Menten assumes a single substrate and a steady state; Langmuir assumes one layer on identical sites with no interaction between them; Stokes-Einstein assumes a hard sphere much larger than the solvent molecules around it. Each is an idealisation that happens to describe real systems well over a useful range, and each fails predictably outside it. Knowing which assumption a number rests on is usually more valuable than the number.