In my opinion, the main aspect that you have to keep in mind when parsing files like these are the structure of the file itself. How it's laid out, since this helps you structure your code so that it can store the needed information. I could give you some advice as to how to actually parse a quake 3 map as I have done that... refer to this thread.
What I have done is did a little research into the Quake/Doom BSP structure and then parsed the file and stored the data needed to actually render the map/model if you wish. Take a look at the code and compare the structure to the structure found here.

I would suggest digging through the X File Format Reference in the SDK Documentation as it will give you alot of information on how to parse the .X file format.

I hope this helps.
Take care.