To me, and based on dictionary definitions a "determinative factor", a "determinant" and a "decisive factor", imply a truely "crucial factor" which can lead you to make a decision about something.

Also, I think I can use the noun "determinant" instead of "a determinative factor":

Determinant:

Something that controls or affects what happens in a particular situation:

• Soil and climate are the main determinants of how land is used.
  

Determinative:

1. able to or serving to settle or determine; deciding
   2. a factor, circumstance, etc. that settles or determines
   

• A determinative factor influences that are determinative of future behavior.

Added;
Decisive:

If a fact, action, or event is decisive, it makes it certain that there will be a "particular result". "Strongly affecting" how a situation will progress or end. An action, event etc that is decisive has "a big effect on the way that something develops"; a "decisive factor"/effect/influence etc.

• The economy is regarded as the decisive factor which will determine the outcome of the general election.

Note: for all these terms, we have only one single term in our language which encapsulates all of these meanings.

I wonder if you could tell me more about these three terms.