problem with 3rd gear?

I have only had my car 2 weeks, bought it from a local dealer, VW Fox 1.2 urban, has 45k miles. I have noticed when I go from 2nd to 3rd gear it crunches and doesn't go into gear sometimes. It only does it from 2nd to 3rd not from 4th to 3rd.
Does anyone have any info on what might be the problem? I have a 3 month warranty on the car and haven't spoken to the dealer yet. Just wondered if it may be something I can sort out myself?
Carol