To plot a function of two variables,z= f(x,y), us the Plot3D command.

Clear[f,x,y];
f[x_,y_]=((4-(x^2)-(y^2))/(Sqrt[Abs[4-(x^2)-(y^2)]]));
Plot3D[f[x,y],{x,-3,3},{y,-3,3},PlotPoints->24,AspectRatio->2,ViewPoint->{10,2,5}]

To get plot of three dimensional equations using the ContourPlot3D command, you must use ContourPlot3D.

Clear[F,x,y,z];
F[x_,y_,z_]=(x^2)+(y^2)+(z^2)-4;
ContourPlot3D[F[x,y,z]==0,{x,-3,3},{y,-3,3},{z,-3,3},PlotPoints->{5,5,5},Axes->True]

Clear[F,x,y,z];
F[x_,y_,z_]=(((x-1))^2)+(((y+3))^2)+(((z-2))^2)-4;
ContourPlot3D[(((x-1))^2)+(((y+3))^2)+(((z-2))^2)==4,{x,0,6},{y,-6,0},{z,0,6},PlotPoints->{5,5,5},Axes->True,AxesLabel->{"x","y","z"},AspectRatio->Automatic,ViewPoint->{10,2,5}]