As a general rule, in teamfights you want to kill the enemy ADC, first as they're the consistent source of damage, followed by the mid laner and other carries, kill the tank last. If the enemy has a fed carry regardless of role, kill them first as an assassin you want to deny their damage source.
Talon and Zed are for more single target burst, and have an easy time getting in and out of team fights. Use your combos on the enemy ADC, if they die wait a couple of seconds until your skills are up again and kill the mid laner.
For Katarina she more so requires her team to engage. Engaging as Katarina puts yourself at a risky spot to be blown upunlike Zed and Talon whom have better escape tools.
If you get exhausted, retreat out wait out the exhaust and try not to use any spells or most importantly your ultimate.
As an assassin the best optimal way to play a team fight is to look for flanks around where the squishy targets are not in good position with their tanks. Not only do you zone away their carries on you but you have the ability to blow one up if done correctly.