This question is a bit vague, but I will try to answer it to the best of my abilities.
Litten
Advantages
resistant to fire, grass, ice, bug, steel, and fairy
Stats consists of a fast physical sweeper (with lots of physical moves)
Final evolution Incineroar resists fire, grass, ice, ghost, dark, steel, and is immune to psychic and is a physical tank
Incineroar gets a dark type move called "Darkest Lariat" and deals damage with no stat change repercussions, whatsoever.
Disadvantages
weak to water, ground, and rock
The stats make litten unreliable as a wall
Final evolution Incineroar is weak to water, fighting, ground, and rock and is slower than the other final evolutions.
Rowlet
Advantages
resistant to grass, water, fighting, and immune to ground
stats make rowlet the wall out of the three, maybe even a "physical" wall.
Its final evolution Decidueye is resistant to water, electric, grass, ground, and is immune to normal and fighting. It is a big special and physical attacker with a massive special defence stat and is the fastest of all three final evolutions. Its signature move " Spirit Shackle" deals damage and traps the foe so they can't escape your wrath.
Disadvantages
weak to fire, ice, poison, flying, and rock
Stats make rowlet not a reliable sweeper, not the fastest neither.
Its final evolution Decidueye is weak to fire, ice, flying, ghost, and dark and isn't as specialized as the other final evolutions.
Popplio
Advantages
resistant to fire, water, ice, and steel.
Stats make Popplio a good "special" wall
Its final evolution, Primarina, is resistant to fire, water, ice, fighting, bug, and dark, and immune to dragon.
Primarina makes a great special tank and its signature move "Sparkling Aria" deals damage.
Disadvantages
Weak to electric and grass
Is outsped by all the other starter Pokemon
Its final evolution, Primarina, is weak to electric, grass, and poison
Primarina's signature move "Sparkling Aria" heals burned targets.
All info is found on Pokemondb.net
hope this helps
