I am firmly against animal abuse. But in the case of the annoying chihuahua in Jigoku Shoujo: Mitsuganae I'll make an exception. It's not because I think that small and incestantly yapping furballs are annoying, though chihuahuas certainly are, but that the series has managed to make animal abuse stop shocking anyone. The people saw that they removed a lot of the underlying story in the second season and for some reason people loved it more than the first, and thought that they could make everything even more monotone in the third season.
My completionist side has got the better of me. I firmly regret seeing the third season of Jigoku Shoujo and want my time back. The writing has devolved into the simplest form, and now we get an angsty magical girl transformation sequence and some fan service to boot. I like fan service, don't get me wrong, but this is supposed to be a "dark" show that takes itself "seriously".
At this point we have had good all the possible combinations of people taking and receiving vengeance, and quite frankly it got tired in the second season (well, the first season to be honest, the second season became painful). By the third series, it is down to recycled ideas made from the earlier recycled ideas of the show. I'll be blunt and say that episodes 2-12 are a complete and utter waste of time and should be skipped. Then, when you think that something will change, we go back to the formula of the first season.
Actually, had this season only had ten episodes, it would have been fantastic. The improved animation is nice. There are the moral grey areas though they aren't explored thoroughly, as nearly everything is wrapped up in one episode.
Writing (Story and Characters):
Even among the rather weak writing of the Jigoku Shoujo franchise, Jigoku Shoujo: Mitsuganae has bad writing. At this point, the staff basically gave up on trying to do something special with the premise. There is no finale as impressive as the second season, and no sympathetic main characters like in the first season. Actually, the ending is obvious, and sucks compared to the one of last season. That is partially because what little arc there is to the season is given a few bites at a time.
Sharp editing is completely lacking from the script department. This season should have been half as long, or even a third as long, without missing anything important. The story of Jigoku Shoujo: Mitsuganae is too boring, too long, and too unevenly paced because of this. So many wasted opportunities to use that extra space to do something interesting, but if you've seen the first two seasons you probably don't care about that anyways. Yes, people suffer, there is revenge, there are morality stories which are aimed at middle-schoolers, and something close to character development. Supposedly everything you need to keep the audience of the series cheering for more. tl;dr: the story is boring and sucks, the editors dropped the ball, but that is the series in a nutshell.
Oh, and the one interesting addition to the cast from the second season is now not a sadist anymore for some unknown reason, but instead comic relief. Character development in the series has actually taken a step back. The characters are somehow even less interesting than last season, which is painfully hard for anything that doesn't aim to make the cast stupid. What little depth characters had last season is removed, and instead focuses on a new character who isn't interesting. It is as if the staff tried to combine what they learned in the first two seasons to take the worst of each and make something bad. Still, they aren't quite worthy of contempt (most of the time), and there are some supposedly "deep" character episodes. You'll forget all that by the end.
Yet another season, luckily the last, of Jigoku Shoujo happened, and as usual, they try and bail out the catastrophy that is most of the writing of the season with a grand ending. Unfortunately, they fail miserably. Jigoku Shoujo: Mitsuganae is the weakest of the three seasons. Now, you may say to yourself "but hey, the first season is rated lower!", and on most sites it is true. Most rational people had the attrocious writing cause them to drop the series either during or after the first season, and the people who rate it are either fans, or completionist freaks like me. I'll tell you this objectively: the series should have ended after season 2, or better yet, with a good editor, all three seasons could have been fit into one and then it would have kicked ass. If there was one editor with some balls in the staff, that would have happened.
Art (Animation and Sound):
As per usual, Jigoku Shoujo: Mitsuganae has slightly improved animation over last season and essentially the same sound.
Overall:
Don't watch this. The improved animation seen in Jigoku Shoujo: Mitsuganae is not enough to cover for the lame writing. If you insist, watch episodes 1, 13, 14, 19, 20, and then 22 until the end. I regret my wasted time and obsessive need to complete any series that doesn't make my brain bleed.