To add to Martok's treatise on how the larger Catholic factions are so badly gutted I'll describe somewhat devious monkey thinking of the AI that lies at the base of such situation.
As a matter of fact, the AI gangs up on factions which are vulnerable enough to be attacked by at least 3 AI opponents and are big enough so as not to provoke a Papal warning. This is precisely why HRE on Early or some other factions in Late (like the French) have such a hard time. On the other hand, factions that start out small never seem to get that much of a beating.
But what to do when you are the biggest in the lot and the others start gnawing at you?
As the fate of many a ruler teaches, never wage war on multiple fronts - that's suicide. Concentrate your efforts on just one opponent, selling your territory as dearly as you can vs the others. When you concentrate against the chosen opponent, head for terminating his royalty. Especially in earlier battles, the AI likes to risk their rulers at some point. If you catch this opportunity, make sure the enemy king dies in that battle and leaves no heir. Rebels are much more complacent
Once you have a wall of Rebels to shield you from one side, regroup your armies and go after another opponent. Usually taking just one of the gang out makes others rethink their behaviour. In my recent French campaign, I was ganged up on by HRE, Spanish and Hungarians. I decided to take out the Spaniards - they always build insane stacks of units, their Jinetes have been always royally p*ssing me off, and the Rebels provide just as good buffer zone against the Almos. In a most fortunate course of events, the Spanish king decided to lead his forces to Aquitaine. And then, my trap sprung off.
In turn 1 I immediately responded with hitting Navarre and Aragon with everything I had. And I have to admit I could throw in forces from literally everywhre, with my fleet almost everywhere and ports in nearly every province. In Aquitaine I retreated my forces to the stronghold, to keep the bad Spaniard king from moving away. In Aragon and Navarre, the Spaniards did the same. Thus, the king was trapped in Aquitaine with nowhere to retreat - God, I love those bottlenecks
On turn 2, I scheduled the castles in Navarre and Aragon to be assaulted, and hit Aquitaine with troops that I had ordered to be built last turn. The Spaniards, however, responded with hitting Navarre with 4-stack army (I had 2 stacks there), apparently trying to unlock their ruler. Too bad for them that Navarre is just a dream to defend: my forces were positioned on such steep a mountain that any enemy force trying to climb up was dispatched almost effortlessly. In the meantime, the battle in Aquitaine went exactly as planned: the Spanish were swarmed and butchered to a man. Before another turn, I was informed that "The Spanish King has been killed in battle. He had no heirs and..." the usual blahblahblah.
Now that I had the Rebel buffer zone and two defensively favourable provinces, I could shift forces against the Hungarians (they had the strongest stacks at the moment). But that war never broke out: the Hungarians got backstabbed by the Byzantines and sued for peace, while the HRE has only limited itself to a small border dispute about Friesland, which it lost eventually. Well, having a 1800-men strong Crusade marching through their land, ready to skin them alive at the faintest sight of offence, must be a royal tranquilizer for some
I have to admit that my empire was already quite built-up by then (fleets everywhere and ability to bring lots of soldiers from almost every province), but the general rule stays in place: kill one opponent's royalty, and the rest goes silent for just long enough to let you catch some breath. The same trick pulled against the English earlier has let me build fleets and structures unhindered for almost 40 years!