Assume I have three quantum states $|u\rangle$, $|v\rangle$ and $|w\rangle$ which can be obtained with three quantum circuits $U$, $V$ and $W$.
We know that we can easily estimate the inner product $\langle u|v\rangle$ or with any of the three states by using an Hadamard or a swap test.
But, what if I wanted to estimate the result of the triple multiplication of the amplitudes of the three states, that is $\sum_i u_i v_i w_i$ (you can assume the amplitudes are real)?
