Answer
Therefore, the interference produces alternating maxima and minima along path 3.
Work Step by Step
We can imagine a screen set up along path 3.
Then this situation is similar to the double-slit experiment.
Sometimes the path length difference to the points along path 3 will be $\Delta L = m\lambda$
Sometimes the path length difference to the points along path 3 will be $\Delta L = (m+\frac{1}{2})~\lambda$
Sometimes the path length difference to the points along path 3 will be in an intermediate state.
Therefore, the interference produces alternating maxima and minima along path 3.
