Plot3D[x^2/9-y^2/4,{x,-40,40},{y,-40,40}]

ContourPlot3D[(x^2+y^2+z^2)^2-4(z^2-x^2-y^2)==0,{x,-2,2},{y,-2,2},{z,-2,2}]

ParametricPlot3D[{2Sin[u]*Cos[u]*Cos[v]/(1+(Sin[u])^2),2Sin[u]*Cos[u]*Sin[v]/(1+(Sin[u])^2),2Cos[u]/(1+(Sin[u])^2)},{u,0,Pi},{v,0,2Pi}]

ParametricPlot3D[{u^2,u*v,v},{u,-1,1},{v,-1,1}]

ParametricPlot3D[{u,v^2,u*v^3},{u,-1,1},{v,-1,1}]

ParametricPlot3D[{3u^4+u^2*v,4u^3+2*u*v,v},{u,-1,1},{v,-1,1}]