Formation of 3-chloromethylbenzene VS 3-chloromethylbenzene VS 4-chloromethylbenzene
It very much depends on the reagents and reaction type you are using to halogenate the aromatic ring, however, the most common method taught during organic chemistry is electrophilic hologenation, which in this case the methyl will be para directing.

http://en.wikipedia.org/wiki/Electrophilic_halogenation#Scope