These words refer to things that are done to deal with a problem.
The most common word isaction. In this use,actionis uncountable.
They are considering legal action.
Acourse of actionis a particular set of things you plan to do to deal with a problem.
They have not yet decided on a particular course of action.
Measuresare individual actions taken to deal with a problem, especially actions taken by a government or official body.Stepscan have a similar meaning.
Measures are in place to prevent flooding.
These simple steps will ensure that infections are not spread.