3/7 is a greater fraction.
Since the numerator is the same, whichever denominator is smaller is the greater fraction.
This is due to the fact that the whole is split up into fewer greater parts, instead of more smaller parts.
A good example is
would you like 1/2 of a pie, or 1/4 of a pie.
We all know 1/2 is greater than 1/4, because each half is greater than just one quarter.