Putting together a list of champions is more art than science. You need to have a sense of what your preferred way of playing is, what the scenario calls for, and (to the extent that you aren't already locked into a list) how to counter your opponent's list. There are so many factors that this will ultimately boil down to instinct and personal preference at some level.
That said, I think there are a few principles which govern how you assemble your perfect trio for each game.
To begin to try and organize my thoughts on which champions can do what, I have assembled an admittedly imperfect table of the champions and what roles I think they can play. My aim here is not today which champions are the best or even the best at any role, but rather what range of capabilities I think they offer. I have broken this down into five categories or "roles." I have some criteria for whether or not a champion can fulfill each role, but I don't always stick to these criteria rigidly. Why not? Various reasons. If you disagree with my table, make your own.
Here are the roles and my loose criteria for each:
Mobility - Can this champion get around the board? I generally say yes if they have the ability to move 5+ nonlinear hexes ini a turn. This includes some champions who get a boost from their followers.
KO Resistance - Does this champion avoid giving up easy KO points? I generally say yes if they have two of the following three things: Dodge+Armor = 6+, 7+ health, some other defensive tech
Pressures Champions - Does this champion have the ability to put 6+ damage dice onto someone? Yes, this is not perfectly consistent with one of my other articles. Thanks for reading.
Pressures Followers - Does this champion (and its followers) have the ability to target 3+ followers in a turn?
Board Control - Does this champion have the ability to meaningfully manipulate the board state? It sounds kind of subjective, but I actually found this one the easiest to adjudicate.
Before you look at the table, please know that I am aware of its limitations. Its purpose is to offer a rough, at a glance impression of who can do what. Raith is the most mobile champion, while someone like Mourneblade is borderline. They both get a "yes" on this table, but they are not equally mobile.
With those caveats, here is the table:
Unsurprisingly, the Guardians are KO Resistant, the Maelstroms pressure followers, etc. What's interesting is where champions cross over to do things you might not expect.
In any list, I try to give myself a range of tools which can answer questions posed by an opponent as well as the ability to ask at least one big question of my opponent. This means assembling a list can both cover all the roles I need a champion to play and at the same time double down on at least one role in order to ask my opponent a tough question.
For example, the list I ran for the Hex and Boon tournament was Lorsann, Rangosh, Mourneblade, and Raith Marid. This list has at least one yes in every column, but also features lots of mobility and lots of champion pressure. In other words, the list can at least try to answer most questions and also can ask at least two: can you keep your champions alive? Can you keep me off your banners? I wanted to run two mobile and hostile champions, so I supported them with a bit of board control.
Generally, I always think you want to ensure the following in a list:
Make sure you can squish flags. This comes down to some combination of mobility and board control.
Make sure you can kill something. This comes down to come combination of follower pressure and champion pressure.
Make sure you have some kind of denial game. This comes down to some combination of KO resistance, board control, and mobility.
Here's are some trios that I would expect to work based on this table, but have not actually tried:
Nia, Finvarr, Peet
Keera, Raith, Grimgut
Morrigan, Rattlebone, Blackjaw
Keera, Rangosh, Shayle
I started off by saying that list selection is more art than science. My hope is that this article has at least helped you to organize your own thinking about how to put champions in particular roles and how to combine those role-players into a super squad. I would be shocked if we agreed about all of the information here, but maybe reading this has clarified your own opinion about list building just a bit.