First you need to make sure that all three fractions have the common denominator which is 24.
3/4 = 18/24
3/8 = 9/24
3/6 = 12/24
Next you compare them and order them from greatest to least. 18/24 is the greatest because the numerator, 18 is the greatest and next comes 12/24 and the least is 9/24.
So if you order them from greatest to least, the answer is 3/4, 3/6, 3/8.
There is another way to solve this which is really simple. If you have the common numerator, even though the denominators are different, you can just compare the denominators. But the fraction that has the smallest denominator is the greatest and the fraction that has the biggest denominator is the least. So the answer would still be 3/4, 3/6, 3/8.