Um, just because you have decided that the solution is what you plotted there, does not make it consistent with the set of inequalities you posed. So you need to tell us what you really wanted to see - the plot that you show, or the solution to the inequalities you actually wrote down.
In fact, I have no idea why you decided that pyramid drawn in your question is correct. In fact, it seimply is not so.
First, consider the pair of inequalities... We have
R1 + R2 + R3 <= 3/2
R2 + R3 <= 2
But as you can see, R2 and R3 are always in the interval [0,1]. So can you see that the second constraint is trivially true ALWAYS? That second constraint essentially does not even enter into the problem.
That leaves the other constraint, that the sum of the three values is no larger than 3/2. I plotted the resulting domain myself, and found that plotregion was indeed exactly correct in what it produces. (Actually, I know what that region does look like, and I know that plotregion got it right, even without redoing the plot using my own software. But I did anyway.)
Perhaps your real question is how can you produce the figure that you actually drew, which is a totally different question, as that would involve a completely different set of inequalities. Not hard to generate them, but I won't bother to do so unless I know there is a reason.