My DS preference is to go to the local Grammar school (RGS) where his brother is already there.
As you live in catchment, and assuming that his brother will still be on the school's roll in September 2013, your younger son will get top priority for a place if he qualifies with a score of 121 or more. He will be in the queue ahead of pretty much everyone else who qualifies, other than siblings who live closer to the school, and there won't 172 of those.
The Admissions Policy for RGS is here: http://www.rgshw.com/library/client/doc ... y_2013.pdf
There is no need for you to name any other grammar schools on your CAF, let alone 3 of them! Please ensure that you tick the sibling box when making your application though.