At weekends, Cannon Street traffic really doesn't need all three platforms, and can easily just use Platforms 1 and 2 or 1 and 3.
Its only in the peaks that this would cause a problem.
True but one additional platform only gives you maybe 3-5 trains worth of extra capacity per hour, and is probably on the low side of that if you have longer turnarounds. It could be an option for the Rainham services, if they can be stepped up approriately.