Plot3D[Sqrt[9-x^2-y^2],{x,-4,4},{y,-4,4}]

Plot3D[Sin[(x+2)/(x+y)],{x,-5,5},{y,-5,5}]

Plot3D[-Sqrt[9-x^2-y^2],{x,-4,4},{y,-4,4}]

Plot3D[y^2/4 + z^2/9,{y,-15,15},{z,-15,15}]

Plot3D[2x+3y+1,{x,-5,5},{y,-5,5}]

Manipulate[Plot3D[{y+x^2+2,z},{x,-5,5},{y,-5,5}],{z,-5,5}]