In just intonation, a pure third (a pure major third, that is) has a frequency ratio of 5:4 between the higher and lower pitches.1
Consider a just tuning based on A4 = 440Hz.
A4 = 440Hz
C#5 = 440Hz * 5/4 = 550Hz
F5 = 550Hz * 5/4 = 687.5Hz
A5 = 687.5Hz * 5/4 = 859.375Hz
However, we expect just octaves to have a 2:1 frequency ratio, so the expected frequency of A5 is 880Hz. That means that (at least) one of our thirds has to be made "wider" — that is, the ratio has to be larger — in order for the octave to be in tune.
Suppose using the above example, we tune C#5 and F5 in 5:4 ratios as shown, but then tune A5 in a 2:1 ration to A4. That would mean that the ratio between F5 and A5 would be 880/687.5, which is 1.28; whereas, 5:4 = 1.25. Thus, a "wider" third.
Put another way:
x Hz * 5/4 * 5/4 * 5/4 < 2x Hz
1 Other ratios are sometimes used, but the principle is the same.