Say I want to plot 

$$E\_1 = \{(x,y,z)\in\mathbb{R}^3\mid 0\le z\le 1-y, \sqrt{x}\le y\le 1, 0\le x \le 1\}$$
or
$$E\_2 = \{(x,y,z)\in\mathbb{R}^3\mid 0\le z\le \sqrt{9-y^2}, {3x}\le y\le 3, 0\le x \le 1\}. $$

Is it possible to accomplish this in Sage? For example, for $E\_1$, I could do something like 

`x,y,z=var('x,y,z');
F = parametric_plot3d((x,sqrt(x),y), (x,0,1), (y,0,1), color='red');
G = parametric_plot3d((x,y,1-y), (x,0,1), (y,0,1),opacity=.5);
XY = plot3d(0, (x,0,1), (y,0,1),opacity=.5);
XZ = parametric_plot3d((x,0,z), (x,0,1), (z,0,1),opacity=.5);
YZ = parametric_plot3d((0,y,z), (y,0,1), (z,0,1),opacity=.5);
show(F+G+XY+XZ+YZ);`

But of course displays each of the surfaces with in the box $[0,1]\times[0,1]\times[0,1]$, not the exact region $E\_1$.