Cry a lot.
In more seriousness, stick to a good peeling support. In the odd case that their team had 3 adc raping champions and your team had no Tank or Peeling support while you played a team reliant ADC, you simply got hard countered to hell.
Situations like that should try to be avoided while in champion select. My go to options for cases like those are Sivir and Kennen. If I feel like I'm going to be focused by 1 or 2 champions (for example, Talon + Jax), Sivir is usually enough with her Spell Shield and Ult. She can also hit the backline while attacking the frontliners thx to her bouncing AA, being allowed to play a bit more safe while doing her job. In the case of a double assassin or 3+ champions made to take me down (ex.: Jax top, Rengar jg, Kat mid), I'd go with Kennen. His E gives him lots of mobility and extra armor and MR, he can double stun most targets (depending on the build, triple stuns are possible), allowing him to either fight back or run away. Imo, he has the best self peel in the game.