Add route to distribute reviews over 3 assignments

As of request by Liedewij Laan for the course nanotechnology

3-way-review can be done via: https://peer.ewi.tudelft.nl/api/distributeReviewsThreeAssignments/[assignment_id1]/[assignment_id2]/[assignment_id3]/[reviews_per_user_per_other_assignment]

Here we have 3 assignments: A, B, and C. The students of 1 assignment review only the other 2 assignments with a fixed number of reviews. So a student from assignment A reviews x submissions from B and x submissions from C. Total reviews for this student: 2x