When thinking about three characters to face a new threat in Hawkins so many people come to mind. Taking Mike, Dustin, and Eddie to go up against this new threat with their close bond, knowledge of monsters, smarts, wit, and strength would be very useful. Having Joyce, Hopper, and Eleven go up against this creature as a family who can help and support each other with power, strength, and wisdom would usually be the perfect combination. But there is a secret trio that no one would possibly think of as the absolute powerhouse they are. I propose the perfect trio to be Eleven, Kali, and Steve.
The obvious choice when it comes to pure power against the upside down is Eleven. She is one of the most powerful kids that were experimented on, being on par at times with Vecna/One who Papa thought was the most powerful of all the kids. Her sheer amount of power and strength can make it easy to go against any kind of threat. The only big weakness she has is her confidence in herself and constantly loosing her power due to that. After she was able to finally close the gate, kill the monsters, all her power seemed to get spent. On top of that, she personally believed that her powers were gone and she was lost. Which is why, in this trio you need to have Kali with her.
In the second season, Eleven strengthened her powers after being with Kali, someone Eleven personally thought of as a sister. Similarly, Kali cared about her like a little sibling and even says that she feels like she has a sister now when eleven spends the night with Kali. The emotional support that Eleven was given, the power she accessed to close the gate was found when she was with Kali. Kali is incredibly powerful herself and has little trouble using it. She is so sure of herself that the worry of their being a mind block that Eleven goes through all the time is not an issue with Kali. The only issue with all that is that Kali is incredibly murderous. Eleven doesn’t like murder as much as Kali, so you need someone to balance her out and keep watch over the two like a mom.
Steve cares about his kids more than anything, will do anything for them and wants to make sure each of them are safe. We don’t always see that with him and Eleven but he still cares about her and only wants the best for her. In the Fandom Wiki Kali is listed as around 20-21 in season two, and Steve is 19 in the latest season. This means she would be about 23-24, so he wouldn’t see her as a kid but someone who cares about Eleven like he does. He would keep her in check in trying to use Eleven in any way and also be able to use his strength to protect them as much as possible.
With these three characters combined, with Eleven having a massive support system in Steve and Kali, with Kali having illusionary powers to pair with Elevens mind power, Steve making sure that Eleven isn’t taken advantage of, and Eleven being able to have Kali to help her use her powers. All of that combined makes for the perfect trio of power, strength, and strategy then nothing could stop them. Granted if there the whole crew with them or even one more person than the group would be even stronger, but just by themselves they are strong. Together they are unstoppable.