Three dimensional pictures may be superimposed using the display
command in the plots package. e.g. suppose the position of a particle
at time t is given by
and we want to observe the trajectory in relation to the planes 
and z=0. We use (note the : used)
with(plots):
a := spacecurve( [ 250*t, (t^2)/8 + 250*t, 250*t - 16*t^2 ], t=0..16 ) :
b := plot3d( {0, (x+y)/10}, x=0..4000, y=0..4000 ) :
display( {a,b} ) ;
Then choose Axes to be Normal, Style to be Patch
with Line Width to be Medium,
and rotate the picture till you get a nice view - 
and
may suit you. Hit p to plot.
Above, the second and third lines define two objects named a and b which contain the pictures. We don't display their results because a and b are stored algebraically (instead of pictorially) and are quite a mess (change : to ; and see what happens). The last line displays the two pictures - superimposed.