What software are you going to be using to create the final md3 model with?
First you need to get the md2 file into your editor of choice, the easiest way is probably using a tool called "Noesis", or if you're going to be using blender, there's lots of different md2 import scrips.
If you go the Noesis route, you can export the .md2 (tris.md2) into a more common format like .obj, then import that .obj into your editor you'll be using to export to .md3.
You'll also have to convert the models UV map from .pcx to a format Quake 3 can read (jpg, tga). Again, if you use the obj method, import your .obj file into your editor, apply the UV map and Mirror the UV map on the Y axis to fix the UV map.
I tried it quick and was able to get it into blender, however you'll still have to re-animate the model for Quake 3. Quake 2 only uses 1 skeleton while Quake 3 uses 2 (upper and lower). You can see inside any player models "animation.cfg" for information on which frames are which for which animation. If you're going to be using blender, you should try an md2 import script first, because it should import the player skeleton and save you the time of making a new one. If not, heres the model in obj format with all 3 parts (head, upper and lower) already pre split up.
Wilma OBJ