yes the answer is r^{3}
the easiest way to do it is as follows, let
a=rcis\theta, \; b=rcis\alpha, \; c=rcis\beta
since mods are all equal. Now
a+bc = r(cis\theta + cis(\alpha + \beta))
since a + bc is real, Im(a +bc)=0, therefore:
Im(a+bc)=r(sin\theta + sin(\alpha+\beta))=0, \\...