Yahari Ore no Seishun Love Comedy wa Machigatteiru. Kan: Dakara, Shishunki wa Owarazu ni, Seishun wa Tsuzuiteiku.

The third season OVA isn't faithful to the "Oregairu Shin" source material. While the main seasons stay true to the original light novel series, the OVA takes a different approach. It selectively incorporates and omits elements from the "Oregairu Shin" light novel, and also adds new scenes that weren't present in the original. You might notice that the OVA is a complete mess if you've read the "Oregairu Shin" light novels. It focuses heavily on Yui's character, while some of the elements that were originally intended for Yukino are either omitted or reduced. It gave everything to Yui that should have been Yukino's. In other ... words, THIS OVA IS JUST ANOTHER STUDIO FEEL'S CORRUPTION!

The OVA covers volumes 1-4 of Oregairu Shin.
Here's what's missing and what's changed in the OVA:

The Date between Hachiman and Yukino is being cut.
All the sweet phone moments between Yukino and Hachiman have also been removed.
In Shin Story, Hachiman completely rejects Yui for being too suspicious, while the OVA original had him resisting the urge to hug her. Additionally, the date between Hachiman and Yui never occurs in Oregairu Shin; this was an OVA original as well.
All the stuff that happened with Yui is well-adapted, while all the moments related to Yukino are left to dust.

The point of Oregairu Shin is to focus on Hachiman and Yukino's relationship. But if Studio "Feel" handles it, Their relationship may not Progress (in the Anime). Don't expect much from them...

And Don't Watch it If you're Yukino Fan, you won't be able to enjoy it. It's a Pain...

Japanese fans mostly favor Yui, so the studio basically made this OVA to promote the VN game, which again exaggerates Yui & Harem's route to get more money, cus why not? Though the canon couple suffers because of this...

This OVA does not stay faithful to the "epilogue" part of the Light Novel known as "Yahari Ore no Seishun Love Comedy wa Machigatteiru Shin." Thus, feel studios should most definitely be criticized for believing that they can make a better ending for this incredible series. However, this isn't even the worst part. Feel studios not only did not stay faithful to the the final light novel, they produced a terrible OVA that ruined many of the characters and left out the scenes that every fan of Oregairu wanted to see from Shin.

This OVA is a disgrace to Oregairu; instead of watching this OVA, ... the reader of this review should read Shin and enjoy the actual final canon of Oregairu.

Oregairu wasn't the same after Season 2 sadly.
First two Seasons were really something else. Oregairu Zoku was a masterpiece with so many great scenes it was so hard to beat by a mile. In the third Season, the direction went in a more romantic approach. However, it felt like the show only had the same name, but lost some of its greatness somewhere between the 5 years gap of waiting for the 3rd Season.
The same goes with this OVA. We have a continuation what happens after the finale of Season 3, but it looks like they just wanted to quickly wrap it up ... as a whole. At least I could enjoy my beloved music and some funny moments.
As a very huge fan of this show, I won't say I'm disappointed, more like a little bit sad they couldn't finish this show with the very last scene from Shin novel. To me, that was the true finale of Oregairu.