Jan gave very good advice. For multiple planes in one graph, Show[graphics,options] would also be needed. E.g.:
Show[ContourPlot3D[x == 2, {x, -5, 5}, {y, -5, 5}, {z, -5, 5}],
ContourPlot3D[y == 3, {x, -5, 5}, {y, -5, 5}, {z, -5, 5}],
ContourPlot3D[z == 4, {x, -5, 5}, {y, -5, 5}, {z, -5, 5}]]