When going for HID's, you don't need to modify anything for the H3's to fit. Well mine didn't.
I don't recommend getting these HID kits because they produce a lot of glare for on coming drivers (it's bad, but not as bad as those who put hid's in reflector housings), they're illegal and you can get defected for them (but what can't these days). They honestly gave me a better light output though. I can now see the road when it's raining.
Edit: I forgot to say something. If you're going to get an HID kit, get 4300K globes with them. I find the 6000K globes to be too blue and ricey looking. I still don't recommend getting them though. Go retrofitting man.