I have not get the question completely, if you want to plot just some points, you can just compute those points! otherwise you can do the following
X = -5:0.01:5;
Y = 2*X.^3-3*X.^2+X;
X_desire = [X(X==-5), X(X==0), X(X==5)];
Y_desire = [Y(X==-5), Y(X==0), Y(X==5)];
plot(X_desire, Y_desire, 'o')
If you want to just put a few label on your plot then
X = -5:0.01:5;
Y = 2*X.^3-3*X.^2+X;
plot(X, Y)
set(gca, 'XTick', [-5, 0, 5])