What surface is represented by the equation z=3 ?

In general, an equation x = k, represents a plane,
parallel to y-z plane.

The equation y = k, represents a
plane, parallel to x-z plane.

The equation z = k,
represents a plane, parallel to x-y plane.

The equation z =
3 represents the set of all points  whose z coordinate is
3.

Therefore, the equation z = 3 is the
horizontal plane, that is parallel to x-y plane and it is located 3 units above x-y
plane.