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Consider the sum $$J= \sum_{m_3=-j_3}^{j_3} \left|C^{j_3,m_3}_{j_1, m_1; j_2, (m_3 - m_1)} \right|^2$$ with $j_1,j_2,j_3\in\mathbb{N}$ and $m_1\in\mathbb{Z}$. For an nonvanishing sum we also need $|j_1-j_2|\leq j_3\leq j_1+j_2$ and $-j_1\leq m_1\leq j_1$.
With these restrictions the sum is independent of $m_1$, so I may set $m_1=j_1$. The sum also increases monotonically with increasing $j_3$, reaching its maximal value $J_{\rm max}$ for $j_3=j_1+j_2$, hence $$J_{\rm max}=\sum_{m=-j_2}^{j_2} \left|C^{j_1+j_2,j_1+m}_{j_1, j_1; j_2, m} \right|^2=\sum_{m=-j_2}^{j_2}\frac{\Gamma (2 j_2+1) \Gamma (2 j_1+j_2+m+1)}{(1+2j_2)\Gamma (2 j_1+2 j_2+1) \Gamma (j_2+m+1)}$$ $$\qquad=\frac{1+2j_1+2j_2}{(1+2j_1)(1+2j_2)}.$$