A level 3 gym is defended by 3 Pokemons. In the video below, the 1,210 CP Starmie defeats 3 defenders and win the gym battle.
In the video, the 1,430 Starmie is active at first and it uses Quick Attack, the normal-type move. The player switches the active one to the 1,210 CP Starmie.
The Hypno uses Confusion, the psychic-type fast move, but it is not effective against the Starmie. On the other hand, the Starmie shoots Water Gun and defeats the first defender.
The Starmie's Water Gun is super effective against Flareon. Based on the type effectiveness, the Starmie has an advantage.
Additionally, the Starmie uses Hydro Pump, the water-type charge move, and the move is super effective against the Flareon.
On the other hand, the Flareon's fast move, Ember, cannot damage the Starmie much. Finally, the Starmie beats the Flareon.
The last defender is the Gyarados, and its fast move is Dragon Breath. Since the Starmie gets injured already, the Gyarados may have an advantage.
The Gyarados uses Dragon Pulse, and the Starmie is defeated. The 1,430 CP Starmie goes to the battle.
The Starmie uses Psybeam, the psychic-type charge move. It is not super effective against the Gyarados but damages the last defender enough to defeat.
Tap the name of each Pokemon to see the best Pokemon to beat it.
The best Pokemon to beat Dragonite/Gyarados [Pokemon Go] | PokemonGo
What is DPS and EPS in Pokemon Go?
Advanced Tips for Catching Pokemon
Best Pokemon to defeat Charizard
Best Pokemon to beat Machamp
Can I catch a wild Mewtwo in Pokemon Go?
How to discard 2km eggs
How to see the journal of your adventure