What can a 3500m runway do for them that the current one can't?
Runway B is currently only 2500m long. I'm not an expert on this by any stretch, but I would think 2500m is inadequate (or not enough safety margin) for quite a few aircrafts to take off, especially with full fuel in suboptimal conditions.
Also, I would think that having a third runway that runs in different direction provides more options in case of cross-winds to the airport which currently only has a pair of parallel runways.