I have played 3V3 a lot in my LoL career, and have noticed that there are some champions that tend to "auto-win" games for there entire team. These are the Champions that you can pick for easy wins.
1. Singed. Singed is the King of 3V3, and until the day he is nerfed, he will continue to give teams easy wins. HIGHLY recommended champion for every team.
2. Jax. Jax is an amazing champion in 3V3 because of his high tankiness, extreme sustain, tons of output damage, and that AoE stun, that also dodges all incoming basic attacks.
3. Darius. Darius can build completely tanky with 1 or 2 AD items and still outdamage almost any other champion. Definitely recommended if you are an unskilled player and are looking for easy champions to win with. WARNING: If you do want to use Darius, be prepared for heavy QQ from the enemy team.
4. Tryndamere. His ult and gap close makes him extremely difficult to kill, and also one of the best tower divers in the game.
5. Kassadin. Although not used very often in 3v3, Kassadin can be a really tough opponent to face in 3v3. He has 2 high scaling AoE abilities, and also a long ranged flash on a low CD. Very safe champion that can also deal tons of AoE damage, silences, and slows.
6. Malphite. Even after the past few nerfs on Malphite, he is still a very strong pick in 3V3. His Q is a hard slow, his W makes turrets (and AD champions) tickle him, his E is an AoE AS debuff that can completely shut down any auto attack reliant champion in a teamfight, and his ultimate... His ultimate is a game changer. Long ranged knockup, that also has a ton of damage.
7. Amumu. Amumu has a perfect kit for 3V3. He has a gapclose that is also a stun, his W provides decent AoE damage, his E gives strong AoE damage, and his ultimate will make winning teamfights a lot less tough, as it is the strongest stun in the game.