I wonder if its possible to make the 5D3 -EV3 like the 6D?
Almost certainly not...
If not then I wonder how they are going to fix this problem
The problem isn't one of sensitivity. Users have reported that while AF is slower with the assist lamp on, simply turning off AF assist results in faster focus - that means even when there's plenty of light as far as the AF system is concerned, AF is slow. So it's an algorithm tweak that's needed, as J.R. stated.