We know that 3.5 is also 3 1/2, and that 3 2/5 and 3 3/8 are both less than 3 1/2.
3.5 must be the greatest.
To convert a fraction to a decimal, divide the numerator by the denominator.
So, 3 2/5 is 3.4, and 3 3/8 is 3.375.
The order is 3 3/8, 3 2/5, and 3.5.