The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
The Power of Three, is the strongest magic in the Charmed Universe, while Willow has been stated (both in show and in the comics) to be the strongest witch in the Buffy Universe.
Unlike to Charmed witches, Willow doesn't require incantations or words to have a spell be done. With the wave of her hand she can warp reality with ease.
The Power of Three, rely on the three being able to recite it together, as long as Willow is able to take one of them out, it is useless. Her best bet would be to go after Phoebe first, as her active powers of Levitation and Empathy (this power could be up to debate giving she can replicate powers) wouldn't have much affect against Willow. Willow is also able to move at much faster speeds through flight, and has been known to have quick reaction feats as well, I don't see why Willow could not flay Phoebe, Paige, Piper, or Prue before they recite the spell. She can also absorb their powers, we know that the Charmed Ones have not done well against beings that can siphon magic, such as when Paige tried to vanquish the Parasite Demons. Of course, the problem with this scenario is going to be Chris, Wyatt and Prue (because she's technically not apart of the Power of Three anymore...right?). Chris and Prue she can probably take him out by throwing him across the room with telekinesis, like with such force it knocks them out for a time being, that leaves Wyatt...and I don't know how that would go to be honest. Willow could also just say "take a nap" and all three of them pass out.
Willow just has too much versatility with her magic and powers for me to say the Charmed Ones would win. She can cast spells from great distances too, so why wouldn't she win.
Also, just to make it clear, this isn't Dark Willow version, this is just Willow, from the comics and everything, canon wise that is.
This is, of course, just my opinion and my thoughts on this battle.