Not only do you have to actively have Thor round up the trio, but there is also a lot of baggage that comes with carrying them to an alien planet and having them join him on his fight against Hela – which would really be the only way to shoehorn them into the story.
Very little of the action takes place on Earth (basically only two sequences in the first act), and that creates certain complications in storytelling.
You do kind of miss them for the sake of continuity and curiosity regarding what they’ve been up to since the events of Thor: The Dark World, but their exclusion is understandable.
If you’re like me, you may walk out of Thor: Ragnarok noticing that the project couldn’t find roles for Natalie Portman, Kat Dennings and Stellan Skarsgard, but you also totally understand why they aren’t featured.