Going after the GC that's in first place is a valid strategy to try to take the lead.

Going after the GC that's in second place is a valid strategy to either extend your lead if you're first place or to take at least second place if you're third.

Going after the GC that's in third place is a valid strategy to extend your lead if you're either in second place or first place.

No matter what GC you are and no matter what place you're in, it's advantageous for the other two GCs to assault you. You will see this, without doubt, as you PvP. It is no conspiracy, nor is it any sort of real plan, it's just basic tactics. And, of course, if your GC is the one that's always assaulted, then you're likely always at 1st place early-midway into a game or have some terrible luck.

Thus is the nature of 3-Way Free For All Faction-based PvP.