Triple Point of a compound and critical point
The essential condition is the compound thermal stability.
If it decomposes below its melting point, it does not have a triple point.
If it decomposes before properties of gaseous and liquid phases converge to each other,  it has just estimated, extrapolated triple point.
Beyond CP we talk about supercritical fluid, it kind of shares many properties of both liquid and gaseous worlds.
It is usually determined experimentally, or as an extrapolation.
See the picture of liquid/gas phase system below, at and above the critical point on Wikipedia: Critical_point