Second Order LP function with three opamps
Look at the path from \$V_1\$ to \$V_{out}\$. We have two integrators cascaded, so:

$$
V_{out}(s)=(\frac{-1}{s\tau})(\frac{-1}{s\tau})V_1(s)=\frac{1}{s^2\tau^2}V_1(s)
$$

After inverting it, we get:

$$
V_1(s)=s^2\tau^2V_{out}(s)
$$

We simply worked backwards from the output.