Given there is a standing wave a 200 and 250Hz, and there are none between 250 and 300Hz, we know that the difference in frequency between each harmonic is 50Hz.
Therefore, the fundamental frequency (the lowest that can be achieved), is 50Hz, as it is 0+the difference in frequency between each harmonic.
The third harmonic is 3x50 = 150Hz, as the 1st harmonic is the fundamental frequency, the 2nd is the fundamental frequency plus the difference between each harmonic and so on.
