The circle you described is two-dimensional. The ‘z’ coordinate is uniformly 0. If you want a circle above the plane, just define a new variable ‘zunit’ and define a vector to define it. This code puts the circle at z=5:
r = 1.25;
x = 0.1;
y = -0.1;
th = 0:pi/50:2*pi;
xunit = r * cos(th) + x;
yunit = r * sin(th) + y;
zunit = 5*ones(size(th));
figure(1)
plot3(xunit, yunit, zunit)
grid on
axis([-1.5 1.5 -1.5 1.5 0 10])
