As you have mentioned in the comment you may have found extrema points of this function. Now you can Identify the extrema which one is minima and which one is maxima .Then you may use this function to plot Minima marked as 'o' and Maxima marked as '*'.
fplot(f) % where f = x^3-x^2-3x
plot(extrema(1), subs(f,extrema(1)), 'o') % extrema(1) is minima
plot(extrema(2), subs(f,extrema(2)), '*') % extrema(2) is maxima
For further reference you may use this document page