Conference paper rejected after positive review and TPC comments
3 reviewers recommended "weak accept," as did the chair, but the paper was rejected.


This is a common situation. Other papers probably got better reviews or your paper was perceived negatively during the discussion phase or ...


  I asked the chair why and he said "your paper didn't make the conference capacity line. Please resubmit to another venue." 


Conferences have upper-bounds on the number of papers they can accept due to time constraints.