Thor: Ragnarok was a huge reset for the Thor franchise, taking what had been the least-liked group of solo movies in the MCU and transforming Thor's story into something that people actually wanted to see. The tone shift was much-needed to give Thor a little more personality, and the bright visuals are very welcome and feel like watching a comic book come to life, which is something that the MCU has ironically been lacking in as far as visuals go. Most would cite it as the best Thor movie, and many would even say that it's their favorite MCU movie in general.
However, despite how good of a movie it is, and how much Taika Waititi's specific vision and style is felt throughout the whole film, Ragnarok is not without its problems. Though the tonal shift was necessary to the success of the movie, it does cause problems in other areas when it's used too lightly. The one instance where this becomes obvious is the way in which the Warriors Three are killed off, with very little fanfare despite being a big part of Thor's story in the earlier movies. Thor: Ragnarok treats these deaths too flippantly, and it really wastes the potential that those characters had.
When Hela comes through the Bifrost and immediately murders Volstagg and Fandral, it feels like the movie is trying to set the tone that she is a huge threat because she can kill these characters so quickly. However, it doesn't just come off as a way to show Hela's power, it comes off as a very emotionless scene that's tossing old characters out the window. The scene might also have been meant to signify that those first two Thor movies were being thrown out, and that the Thor series was going to be very different going forwards. To get this message across by showing no empathy or respect to these characters who were such a big part of Thor's story initially, however, makes the message feel like a cheap shot more than anything else.
It would be one thing to kill off the Warriors Three in Ragnarok in order to create an emotional scene or to give some motivation to Thor to defeat Hela, but it's quite another to kill them off incredibly quickly and with no real reaction from Thor. Hogun is the only one who gets to really fight back in any sort of way and get a real death scene, but the appearances of these characters feel like cameos more than anything else. The audience might watch the entire movie and not even know that Volstagg and Fandral were on screen, because they get about one second of screen time before Hela kills them.
Sure, maybe the Warriors Three weren't given much to do in the first two Thor movies, but that's something that could have been rectified by Ragnarok. Even if they didn't want to include the Warriors Three and wanted to expand Thor's world instead (by introducing new recurring characters to his story like Valkyrie and Korg), they could have left the Warriors Three to be dealt with in another movie, and have them come back and be really important. The characters have the potential to be fun, and killing them off unceremoniously is such a waste of that.
Their death scenes could have been impactful if they had impacted Thor's story in any way, but he doesn't even really acknowledge what happened. It's like he doesn't even know about it, which is, again, such a waste of a potentially good plot point. Learning that Hela murdered his best friends would be a great motivation for Thor to want to fight her (even though he already had reason enough), and would just add some more much-needed emotion to the story. The stakes would feel so much higher if the movie took a more serious tone when it needed to and actually acknowledged the gravity of the things that happen in it.
Even when Asgard is destroyed, it doesn't seem like that turn of events is given the weight it deserves. They almost make a joke of it, which is strange because this should be an incredibly sad moment. All of these characters are watching their home get destroyed, and it doesn't seem to phase them, particularly Thor, that much. Asgard may be "a people, not a place", but that doesn't lessen the emotional attachment that these characters would have to their home. While throwing out some of the elements of the first two movies to give Thor a fresh start seems like a great idea, it doesn't quite work to its full extent if those elements are treated like trash rather than important parts of Thor's story.
In short, it's really disappointing that Thor: Ragnarok handled the deaths of the Warriors Three the way it did. It was such a lazy and clunky way to write them out of the story, and it doesn't even give them any sort of agency or dignity in their deaths. It's fine to take a lighter tone, but when that involves turning the deaths of somewhat major characters into a joke, it suddenly doesn't seem like such a good idea.
There was a way to go about it that would have respected the characters but still launched Thor into a new phase, but unfortunately, Ragnarok wasn't able to strike that balance. There was a lot of potential to develop those characters more and actually have them be more impactful to Thor's story since they're a part of his crew in the comics, but unless the MCU starts using the multiverse to bring characters back from the dead, it's unlikely that fans will ever get the chance to see that happen.