The blurb is just dishonest:
_To put their demons to rest once and for all, Josh Lambert and a college-aged Dalton Lambert must go deeper into The Further than ever before, facing their family's dark past and a host of new and more horrifying terrors that lurk behind the red door._

In order of the claims:
They don't go deeper into The Further than ever before--they literally go to the same places they've already been, and not as deep as they have been in previous movies.
They faced more of the family's "dark past" in the second installment. In this one, they only revealed one thing that's barely a spoiler: [spoiler]Josh's dad could also astral project into the further and was really upset about it. But not enough to actually matter to the plot.[/spoiler]
There wasn't a "host" of new and more horrifying terrors. First of all, there were only two "new" terrors, who turned out to [spoiler] not really be terrors at all. Just a puking ghost who was upset about the door being open for no explained reason and Josh's dad.[/spoiler] and the red-faced demon again. The main antagonist was literally just the same antagonist again.

The movie merely rehashed the same villains, discoveries, and threats faced in the earlier entries because of the incredibly useful trope of "forgetting" that the previous movies happened. It had a promising start which is the only reason I've generously given it a 4, but it very much did not deliver.