Another way to get to the solution, no guessing needed. No spoilers as two solutions were posted already.
There are 13 lines. This means one officer needs to cover 4 lines and remaining 3 policemen need to cover 3 lines each, assuming no overlap. There are 3 places with 4 lines (and none with more than 4): top left, mid left and the mid near bottom. All three places see each other - if we place officers on two of them, they together still cover just 7 places (as if one spot had 3 lines), meaning it is not possible to place an officer on a spot overseeing just 2 lines or a spot with 3 lines seeing one of our policemen.
This already dramatically decreases number of possible spots for our officers. We could start guessing, but we can do better.
Observe bottom line. Out of theoretically 5 spots for policemen we are down to just 2 corners with 3 lines each, other spots have just 2 lines and can't lead to any solution. Bottom left corner sees all places covering 4 lines. This would mean as if it had 2 unique lines to add. Meaning the bottom right corner has a policeman as it is the only one remaining.
That policeman sees 2 spots with 4 lines. Those are eliminated, meaning another policeman is in the top left corner. Just 2 spots with 3 lines remain - put the remaining two policemen there for our unique solution.