The volume of the real flag manifold $Fl_R^3 = O(3)/Z_2^3$ can be obtained on one hand by the explicit integration
on the $O(N)$ invariant volume element on its big cell:

$\int_0^{\infty}dx_1\int_0^{\infty}dx_2\int_0^{\infty}dx_3(1+x_1^2+(x_3-\frac{x_1x_2}{2})^2)^{-1} (1+x_2^2+(x_3+\frac{x_1x_2}{2})^2)^{-1}$

This integral is not hard to evaluate using elementary integration techniques.
The result $2\pi^2$ can also be obtained from:
$Vol(Fl_R^3)= Vol(RP^1) Vol(RP^2) = \frac{Vol(S^1)}{2}\frac{Vol(S^2)}{2}$