Terrakion is one of the strongest wallbreakers in the UU tier thanks
to its high Attack stat and amazing STAB combination. It can also work
as a fearsome setup sweeper with Swords Dance, as it's able to
pressure and break through a lot of stall and balance teams. Despite
facing competition from other Fighting-types like Infernape and
Cobalion, Terrakion sets itself apart from them due to its ability to
beat common Fighting-type answers like Togekiss and Moltres, its
better Attack stat, and its access to Justified. However, Terrakion's
typing is very poor defensively, making it easily exploitable and weak
to a lot of priority moves like Bullet Punch, Mach Punch, and Aqua Jet
and common typings, such as Steel, Psychic, Ground, and Fighting. The
Defense and Special Defense drops caused by using Close Combat only
aggravate this weakness. On sets utilizing a Z-Move, Terrakion can be
easily exploited by bulky threats like Hippowdon and Alomomola that
can tank a hit from it and cripple it after it has used its Z-Move.
Terrakion is also outsped by a lot of common threats in the metagame,
such as Latias and Gengar, meaning it can have a hard time sweeping.
Terrakion must wasnt the best but not the worst, due to it super frail typing but pairing with a fine stat.
Keldeo's solid Special Attack and Speed stat allow it to function as a
solid balance breaker in the OU tier. A Water / Fighting typing in
conjunction with solid bulk for an offensive Pokemon allows Keldeo to
check threatening Water-, Fire-, Steel-, and Dark-type Pokemon such as
Ash-Greninja, Heatran, Mega Scizor, Tyranitar, and Weavile. Keldeo's
STAB Secret Sword also allows it to bypass traditional checks to
special attacking Water-types such as Chansey and Ferrothorn.
Unfortunately, not all shines for Keldeo, as numerous Pokemon such as
Toxapex, Amoonguss, and Tapu Fini are all hard counters to most Keldeo
sets. Keldeo also lacks any powerful coverage options and is forced to
rely on weaker moves such as Hidden Power and Icy Wind.
Keldeo was almost as same as terrakion, but a special one when lest weaknesses. However, it still got bunch of weakness. It got a fine movepool also.
Tornadus is a versatile offensive Pokemon that can function as a wallbreaker with its solid power and coverage or as a utility pivot with U-turn and Prankster status moves like Defog and Tailwind. Its high Special Attack and high Speed allow it to outspeed several prominent threats in UU, including Gengar, Celebi, and Infernape, and overpower them with STAB Hurricane. It also can utilize Supersonic Skystrike to even break past bulkier offensive threats like Latias and Terrakion after minor chip damage. This middle ground of power and Speed allows it to distinguish itself from other Flying-type choices. Its large range of coverage options also benefits it, allowing it to utilize moves like Superpower and Heat Wave to greatly limit its switch-ins. Unfortunately, Tornadus provides very little defensive utility to teams due to its Stealth Rock weakness, lack of recovery, mediocre defenses, and lack of useful resistances. Because of this, it faces competition from other offensive Flying-types such as Moltres, Mega Pidgeot, and Togekiss, which have better defensive typings, better bulk, and access to reliable recovery. Its reliance on Hurricane as its STAB attack can be disadvantageous due to the move's poor accuracy. Lastly, due to Tornadus's poor defenses and typing, it struggles to do anything of worth to most priority users like Mamoswine and Scizor, as they can power through it easily after minor chip damage.
It was the wrost in these three, despide on its stat wasnt as good as other flying type and and its got a poor bulk and typing with super less resistense.
Hope it helps~!
And also, you might want to check https://www.smogon.com/ for compective Pokemon playing and https://pokemondb.net/pokebase/348721/how-tell-pokemon-outclasses-another-competitive-battling for learning about compective Pokemon guide! Good luck on your Pokemon Learning Journy, Ive experienced it before too :)