How to project 3d image in the planes xy, xz, yz?

Update 2: The function projectToWalls does not work in version 12.0 because the function PlotRange no longer works. To fix the issue, replace PlotRange with plotRange where

 plotRange = PlotRange/.AbsoluteOptions[#, PlotRange] &;

in the definition of projectToWalls.

Original answer:

You can post-process a Graphics3D object to project the lines to the left, back and bottom planes using a function like:

ClearAll[projectToWalls]
projectToWalls =  Module[{pr = PlotRange[#]}, 
    Normal[#] /. Line[x_, ___] :> 
       {Line[x], Line[x /. {a_, b_, c_} :> {pr[[1, 1]], b, c}], 
       Line[x /. {a_, b_, c_} :> {a, pr[[2, 2]], c}], 
       Line[x /. {a_, b_, c_} :> {a, b, pr[[3, 1]]}]}] &;

Examples:

pp1 = ParametricPlot3D[{{4 + (3 + Cos[v]) Sin[u], 
        4 + (3 + Cos[v]) Cos[u], 4 + Sin[v]}, {8 + (3 + Cos[v]) Cos[u], 
        3 + Sin[v], 4 + (3 + Cos[v]) Sin[u]}}, {u, 0, 2 Pi}, {v, 0, 2 Pi},
    PlotStyle -> {Red, Green}];

projectToWalls @ pp1

projectToWalls @
 Graphics3D[{White, MeshPrimitives[Tetrahedron[], 1], 
   MeshPrimitives[Cuboid[{0, 1/2, 0}], 1]}, 
  PlotRange -> {{-1, 2}, {-1, 2}, {-1, 2}}, Background -> Black]

Update: Taking Roman's idea a step further using Textured polygons:

SeedRandom[1234];
P = Graphics3D[{Hue@RandomReal[], #} & /@ Cuboid @@@ RandomReal[{0, 1}, {10, 2, 3}]];
pr = PlotRange[P];
rect = {#, {#2[[1]], #[[-1]]}, #2, {#[[1]], #2[[-1]]}} & @@ Transpose[pr[[{##}]]] &;
texturedPoly = {Texture[Rasterize[#, Background -> None]], 
    Polygon[#2, VertexTextureCoordinates -> {{0, 0}, {1, 0}, {1, 1}, {0, 1}}]} &;
{left, back, bottom} = Show[P, ViewPoint -> #, Boxed -> False, Axes -> False, 
    Lighting -> "Neutral"] & /@ {Right, Front, Top}; 
leftWall = Prepend[#, pr[[1, 1]] - 1] & /@ rect[2, 3];
backWall = Insert[#, pr[[2, 1]] + 2, 2] & /@ rect[1, 3];
bottomWall = Append[#, pr[[3, 1]] - 1] & /@ rect[1, 2];

Graphics3D[{Opacity[.2], P[[1]], EdgeForm[None], Opacity[1], 
  MapThread[texturedPoly, {{left, back, bottom}, {leftWall, backWall, bottomWall}}]}, 
  BoxRatios -> 1, PlotRange -> {{-1, 1.5}, {-.5, 2.1}, {-1, 1.5}}]

If you only need the 2D projection images, you can just project the 3D image from the six cardinal directions:

SeedRandom[1234];
P = Graphics3D[{RandomColor[], #} & /@ Cuboid @@@ RandomReal[{0, 1}, {10, 2, 3}]]

Show[P, ViewPoint -> #] & /@ {{∞,0,0}, {-∞,0,0}, {0,∞,0}, {0,-∞,0}, {0,0,∞}, {0,0,-∞}}

Working with the ViewVertical option might also help.