Each starter is one of the best Pokemon of their respective types, so you could definitely use all three to great effect. The combination of Grass, Fire, Water, Ghost, Dark, and Fairy STAB hits twelve types for super effective damage, which can be further increased by rounding out the rest of your team to cover the types these three Pokemon can't.
Decidueye completely clowns on the two Ula'ula Island trials, with a resistance to Totem Togedemaru and STAB Ghost moves against Totem Mimikyu. In addition, it can completely solo Hapu's Grand Trial, plus it's a good Pokemon to bring to the Elite Four thanks to its Fighting immunity and Ghost STAB. Dartrix also offers decent matchups into Totem Wishiwashi and Totem Lurantis.
Incineroar's STAB combo doesn't hit as many important trainers super effectively as the other two starters, but it's arguably a better answer to Acerola than Decidueye is, thanks to its Ghost resistance and super effective Dark STAB. Its Fire moves also do well into Guzma (minus Golisopod), Totem Ribombee, and Molayne. Torracat puts pressure on Totem Lurantis effectively as well.
Popplio is really good into the entirety of Melemele Island. The fight against Ilima can be iffy, but it does well against Totem Raticate with Disarming Voice and Growl. It can do the exact same thing against Hala's Grand Trial. Brionne is a fairly consistent Pokemon to bring to Totem Marowak, but be mindful it won't have any good Water moves except Aqua Jet at that point. It's great against Olivia, Nanu, and Hapu, which makes it easily the best starter for the Grand Trials. Its 4x effective Fairy STAB answers Totem Kommo-o and it achieves good results in the Pokemon League as well.
All three starters have a super effective STAB move against Ultra Necrozma, so using all three as needed should trivialize one of the hardest Pokemon in the game. Do note that all three starters are somewhat slow, so some healing items may be necessary every now and again.
Overall, each starter brings a unique set of strengths to the table, and each one will help you on different legs of your journey. Their powerful STAB moves will serve you well -- as long as you mind their weaknesses.
