Perdan's influence is the strongest in my opinion. Sirion looks big but Sirion only makes as much income as Perdan. As for Caligus, they have significantly been weakened due to Dunnera. They have about the same number of nobles as Westmoor while having more regions to govern. But their diplomatic position is strongest among the big three, allied with both Perdan and Sirion. If either Perdan or Sirion want to go to war, they need to lower their relations with Caligus first which is not something they neither of them are willing to do first.