Is The Power of Three weak/not that good?

Power of Three is character driven and has no real plot. It's mainly a set up for the fourth series.

If you like the characters, you should be fine.

I stopped reading Warriors years ago because of the content of The Power of Three. Having read it through all now, I'd say it's better than you might expect, as long as you bear in mind the issues with the plots of the books - there are three main perspectives, so that gives us some more insight we didn't get before. I'd give it a shot. If you do, you're best reading the fourth series (which most people rate lower than the third) because it ties up a lot of loose ends, since the third arc sets up for it. It took me two books to get comfortable with the main cast though, so bear in mind there's something of a slow, weird start to the first book...

Yeah PoT is more of a lead-in for OoTS's plot rather than having an actual plot itself. It's more character-driven than it is plot-driven.
Though to be fair, that's NOT entirely a bad thing. While some complain about the pacing, if you prefer character-focused stories than plot-focused ones, then the slow pacing this arc has shouldn't be too much of an issue at all. I myself find character-driven stuff more appealing to read, rather than plot-driven where characters seem to ride the plot. The only book that is an absolute struggle to get through in this series imo is "Outcast", it to me is a slog to get through. And "Dark River" can be annoying to read as well because so much BS happens in it for such stupid things. What exactly? Well you said no spoilers so I can't really tell you.

I personally consider TNP better than it, but it itself not too bad a read either imo. It's somewhere middling (But for me it's still my least favorite that I've read).
Like said, it's more character-driven and doesn't really have a plot, as it's just a lead-in for OoTS's. So if you're fine with the books focusing more on the characters, and a slower pace, then by all means go on ahead.

It's my least favorite arc :/

it’s not the worst series, but it’s really not amazing. as brindlefern said, it’s more of a lead-in to OoTS. it has pretty much no plot and the only reason i tolerate it is because i like the characters.

The Power of Three is my absolute favorite arc of the entire franchise. For me it has the mos engaging characters, and no other arc has really made me feel for them or want to know more about them.

However, PoT does have an unsatisfactory ending for many without real closure, and for readers who prefer a linear narrative, PoT does not have that. PoT is not plot driven, it is character driven, and so to appreciate PoT for what it is, you have to set aside the plot of impeding doom and look instead at how this effects the characters within the narrative. We take a look into their psyches and really get to see flawed, more humanistic characters before. Whereas the Original Series has a very black and white moral tone to it, and is traditionally carried by heroic archetypes, and TNP is almost solely plot driven and very little character is developed, PoT completely subverts this by making what one might consider 'good' to be in the wrong, and what one might consider 'evil' to be sympathetic.

PoT's biggest issue is that it was a set up for Omen of the Stars, which would have the real plot driven payoff. PoT is the tense, gradual calm before the storm, and we see throughout the books the characters put through worse and worse situations, seeing in full depth just what it feels like to be a main character, and it isn't always good.

So, the lack of a payoff, increasing tonal change to get darker and darker each book, and almost complete focus on character driven story rather than plot driven story may turn people away from PoT. As for me, its my favorite series.

Not hard, considering TNP is actually trash.