Let \( a_1, a_2, a_3, \ldots \) be a G.P. of increasing positive terms such that \( a_2 \cdot a_3 \cdot a_4 = 64 \) and \( a_1 + a_3 + a_5 = \frac{813}{7} \). Then \( a_3 + a_5 + a_7 \) is equal to:
\( 3256 \)
\( 3252 \)
\( 3244 \)
\( 3248 \)