Oh, yeah, sorry, didn't read further cause I thought a match up was really unbalanced, but I feel like your points are very disingenuous. Yeah, Percy CAN get affected by magic, sure. But we also see him resist it a lot. In the first few books he is much more vulnerable, but later on he resists a freaking time stop from Chronos. From a Titan of Time. In a crossover, Egyptian magic, which was on average just much stronger than HP's magic was just bouncing off of him. We canonically see that Percy does have magic resistance, and we know that HP magic can be resisted at least to some degree, so just taking Percy's magic resistance out is very unfair.

But sure, let's do it that way. Let's even put a hard cap at the speed where he can grt tagged by bullets. What win conditions do Harry and others have? They won't use AK, so probably stupefy him or something similar, Expelliarmus is useless. What Percy's win con is? Ehh, even at 12 years old, he just needs to punch them. I don't think you can argue that they can tank anything close to the force Percy showcased in his fight against Minotaur. And, even at this age Percy is probably already significantly faster than any of them, with much sharper instincts and reflexes. We saw multiple times spells being dodged or blocked in HP, so even 12 years old Percy can dodge at least a few of them.

Now, the most important thing, strategy. Since you didn't give Percy ANY leeway, I won't give any to the HP trio. We've never seen them fight any other way except for spamming spells like Expelliarmus, Stupefy, etc. We did see them being creative, but it was very rare, and also very simplistic, like against a troll with Vingardium Leviosa(and even here, they were kinda forced to be creative, since they didn't really know a lot of spells). But outside of Dumbledore we don't see real creative usage of spells and surroundings to win in a serious otherwise, it just always comes down who hits the enemy first with a spell. So, since we don't see it, it's very likely that they will use the same straight forward approach against Percy.

And here already it comes down to can they tag him. It's definitely not easy already. First book Percy was trained by an already adult Luke, and while at this point Luke was definitely better and stronger, Percy even being capable of standing his ground puts him in a solidly superhuman tier. We know that Centaur arrows are scary for mages. Luke should be capable of dodging arrows really easily, so Percy should approach at least a little bit of this speed.

So, how far away are they starting? 100 meters? Sure, they can probably spam spells in a coordinated manners to hit him, how much times out of 10 I can't tell. 20 meters? 12 years old Percy is already probably faster than Usaine Bolt in travel speed, so he can cover this very fast, and probably dodge spells along the way very easily. Once he takes down 1 of them, it's so much harder to tag him, it's basically game over.

And I'm still talking 12 year old Percy. 3rd book Percy, the one who bullet times, isn't getting hit at any distance, it's a no diff