There are up to 8 tracks east of Clapham Jn, but the Windsor side and the Mainline side are normally operated separately. The problem the London and SE RUS tries to address is the lack of capacity on the fast lines inbound from Woking through Wimbledon and then Clapham Jn to Waterloo. A number of medium distance services run on the fasts through Wimbledon and switch to the slow lines further out beyond Surbiton, and they want to be able to run these on the slows all the way into Waterloo. Amongst other advantages it allows far more trains to stop at Clapham Jn, as discussed in previous threads. That means the existing users of the slows would have to be diverted - and CR2 is a useful option for that - by effectively providing six tracking inbound from Raynes Park.
Putting your CR3 idea onto the slows between Waterloo and Clapham Jn removes the advantage CR2 provides for SWML long distance services.
These services actually switch to the slow lines immediately before Surbiton in order to call there.