HMMT February

Let $S$ denote the given sum. By summing over all six permutations of the variables $a, b, c$ we obtain $$\begin{aligned} 6 S & =\sum_{a=1}^{\infty} \sum_{b=1}^{\infty} \sum_{c=1}^{\infty} \frac{3\left(a^{2} b+a^{2} c+b^{2} a+b^{2} c+c^{2} a+c^{2} b\right)+6 a b c}{4^{a+b+c}(a+b)(b+c)(c+a)} \\ & =\sum_{a=1}^{\infty} \sum_{b=1}^{\infty} \sum_{c=1}^{\infty} \frac{3}{4^{a+b+c}} \\ & =3\left(\sum_{a=1}^{\infty} \frac{1}{4^{a}}\right)\left(\sum_{b=1}^{\infty} \frac{1}{4^{b}}\right)\left(\sum_{c=1}^{\infty} \frac{1}{4^{c}}\right) \\ & =3\left(\frac{1}{3}\right)^{3} \\ & =\frac{1}{9} \end{aligned}$$ Hence $S=\frac{1}{54}$.