What you would need to do to compare these fractions would be to change them so they share a common denominator. The easiest way to do this is to find the lowest common denominator. For these fractions, it would be 40.
3/4 becomes 30/40
7/8 becomes 35/40
1/5 becomes 8/40
So if you were to order them least to greatest it would come out to:
1/5, 3/4, 7/8