The second step towards the solution is understanding that the given three point images determine only the position of the intersection point F of a focal plane with the line s connecting the three given points: a lense with arbitrary orientation and arbitrary focal length does satisfy all the conditions of the problem as long as its focal plane intersects with the line s at the point F.
Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. The next hint for the Problem 4 will be published at 13:00 GMT, 13th March 2022, together with the next updates of the intermediate results.