Olaf, Darius, Singed - One mistake against them loses you the game. If you don't end the game soon enough you still lose.
Wukong, Amumu, Rumble - I went against this once and it was frustrating. You can beat them early, but but they will come back hard and one mistake and it's over. If they ace you at any point you might as well just surrender. One of them presses R, then they all press R and tons of damage is dealt.
Lux, Ezreal, Malphite - Once they all ult, you pretty much have to back up because most of your health bar is gone. So much CC, mobility, and poke. They also have really annoying alter control.