First you read this guide, you may start to laugh, but after trying this, it wont be funny any more. In 3v3 most of the teams come with 3 tanky champs, so you have only chance if you come with tanky too, but not any more, here is the setup. After getting madred, your team is unstoppable. Banning: Shen, Morgana and Mordekaiser, they can counter you. You have to care in the beginning, until you dont have the madred, you have no dmg. So you have to care to farm well, and dont give free farm to the enemy, with the razors, you can get the dragon and the jungle easily. If there is no jungler-> vayne top, keni and teemo bot. In the case of jungler keni and teemo go up, and vayne stays bot. Cuz of the top lane can be ganked in 4 ways, but the bot only 2 ways. If you do well, you can push them to the turrets, and dont give them any kind of farm(kills, minions). Cuz you are ranged you can easily push and destroy turrets. Teemo's shroomys can provide free wards, and safe for you, putting the whole mid with them. Focusing the biggest dmg, and so on, so the most important is the focusing, as usual. Kennen have to start the teamfight, cuz of tons of stuns, then sometimes, he dies, but till that, you will won the team fight.
Enemy cant build defensive, cuz you have 1 ad and 2 hybrid dmg. But if you do well, they cant build anything.
In the case of loosing, you can win the match with the last turret too, putting teemo's shroomys, and the minions and the enemy cant come to your turrets.

Runes are given. Teemo and Keni is full attack speed. Vayne attack dmg.

Vayne the usual carry masteries. Teemo and Keni is tanky and a bit offensive masteries.

Keni and Teemp madred, attack speed and hp.
Vayne pure ad carry build.

The heal is needed, sometimes you are burst, and the 3 heals, can give you one more chance.
The exhaust is need if there is at least 2 ad, but you can use flash instead of it, if they have 2 mage, or only one ad.

In the beginning you can feed the enemy, cuz you have no dmg and hp. But after getting madred, with nice farming, and getting the dragons, its easy.