I have seen this organic compound named in two different ways.
According to some sources it is written pentan-3-one.. Other sources call it 3-pentanone.
Which one is correct?
Whether you write pentan-3-one or 3-pentanone both are correct.
But according to IUPAC rule pentan-3-one this is correct format of writing the name of organic compound. So I prefer you to write pentan-3-one rather than 3-pentanone.
You should also read this.
Adding to Freddy's answer I would say that, pentan-3-one is preferred overall due to the fact that as molecules start to carry more and more functional groups, it becomes ambiguous of which functional group the number in the front is referring to without knowing the order of priority of naming functional groups.