Why is Plagueis at #3?
Because he's just that good.
He's strong enough to punch through the armored torsos of Maladians, and his kick sent a Zabrak flying clear across a cabin, snapping his spine, and bursting his arteries.
He's fast enough that 11-4D couldn't detect his movements, and 11-4D was a high-end droid designed to track and record blaster bolts. He also noted that during his battle with Venamis, all an onlooker would see is two streaks of red lightning flashing through the forest.
His telekinesis is strong enough that his waves carry enough force to rip apart targets on a molecular level, blowing a squad of Maladians into invisibly small pieces with powerful shoves. (Investigators believed that the Maladians were killed by heavy explosives after the fact.) He also casually ragdolled Darth Sidious while the latter was still an apprentice.
As for other applications of the Force, his Force Lightning was powerful enough to turn it's target's bones into dust, and his FLS was of great enough magnitude to light up an entire landscape. He can also increase his own power for short periods by activating his own Midichlorians to their fullest potential, or by increasing the count held in his body.
He's not lacking in skill, either. Darth Tenebrous references Plagueis as a master duelist despite his distaste for the arts, and let's not forget that Darth Venamis--whom not only was Plagueis was able to defeat before his peak, but also was one of the most unpredictable fighters I can recall--was himself a master duelist of great enough calibre to switch and mix lightsaber styles and techniques while in the midst of combat. Despite having been trained specifically to counter Plagueis's style of combat, he was still bested.
Not only are his combat abilities magnificent, but he's also one of the hardest opponents to kill. His skin's force-enhanced toughness alone was compared to stone, the claws and teeth of two Kaleesh unable to even penetrate his skin, and with Midichlorian Manipulation he was granted similar abilities to Dark Transfer in the sense that he can bring beings back from the brink of death and efficiently heal even dangerous injuries. If all else fails, he also can perform cardioversions through the Force on any of his 3 hearts, and thus can control his own blood output and extend his life to continue battle despite having grievous wounds.
Plagueis would also supposedly have bested Darth Sidious had he challenged him to combat in TPM rather than murdering him underhandedly, and that speaks for itself. He's just top tier, I would agree with Carthage's placement of him--He's probably #3, or possibly even #2.