At the end of the first week, the other plants had been 1, 2, or 5 centimeters tall. These other plants all doubled in height. What were the new heights?
At this point, I believe you're left with guess-n-check for finding a combination that will work, since there isn't enough information to form a system of equations that can be solved for a unique value. One way to start would be with values "in the middle", such as 10 of the shortest height, 9 of the middling height, and 10 of the tallest height. Then adjust until you can find a combination that works.