the weekly treatment makes a lot more sense to me. Cisplatin enhances the radiation effects. Since you are getting that every day it makes more sense to me to keep the chemo level more constant (i.e. weekly).
So:
A) more constant drug concentration and less of a spike
B) better chance to intervene if something happens, i.e. oto- or nephrotoxicity
C) The overall amount you get is about the same (more or less)
D) you are more likely to finish the 7 treatments than the 3 big ones. Quite a few people only had 2 treatments.

As with everything you get here discuss with your MO. It seems that the 3 treatments is something the used to do in the past..

M