f=(x^2+y^2)

Plot3D[f,{x,-3,3},{y,-3,3},PlotTheme->ZMesh]
ContourPlot[f,{x,-3,3},{y,-3,3},Contours->{2,3},ContourShading->None,ContourLabels->True]

ContourPlot3D[x^2+y^2+z^2,{x,-3,3},{y,-3,3},{z,-3,3},Contours->4]