You don't have many points in t. Try using linspace and specify a larger number.
t = linspace(0, 10*pi, 2000);
xt = sin(t);
yt = cos(t);
plot3(xt,yt,t,'-o','Color','b','MarkerSize',10)
hold on
t5 = 5;
xt5 = sin(5);
yt5 = cos(5);
plot3(t5,xt5,yt5,'MarkerSize',200,'Color','g')
xt = sin(5*t);
yt = cos(5*t);
plot3(xt,yt,t,'-','Color','r','MarkerSize',10)
hold off
axis equal
xlim([-1, 1]);
ylim([-1, 1]);
zlim([0, 5]);
