Find a common multiple in between the denominators.
10 and 3 and 5 -> the least common multiple is 30.
Change all the denominators to 30 by multiplying the top and bottom by the same number.
9/10=27/30
1/3=10/30
1/5=6/30
Subtract!
27/39-10/30-6/30=11/30
The answer is 11/30
Hope this helps!