I love 3v3. A big thing to note is that quick communication is key, due to the small size of the map. Top may be going to the jungle, so u need to let them know that they r rotating so y'all can counter gank.
I have noticed that many people with put 2 champs in a lane and skip having a jungler. While having a jungler seems useless with such a small map, I recommend having one. As long as someone on ur team can handle 2 people without overextending and getting frequent ganks from ur jungler, u should be fine. If ur having a tough game, stealing jungle is handy to starve the enemy as well as give u extra gold.
I've noticed that miss fortune, renekton, Galio, mao, and j4 are very good. There r many but those r the first that come to mind.
There's a new item, I wish I knew the name of it at the top of my head, that helps u so long as u get ur alter. It makes u look like ur on fire I think. When the alter is a red hexagon, u can get it. More than one person captures it faster. Just be careful of the enemy jungler if they r roaming into ur jungle. Also, stealing the enemy's alter helps u too. Oh! If the enemy hits u while u r in the process of capping it, it interrupts the cap.
The "baron" is called vilemaw. U want all three people there, preferably, to kill it faster, preferably after u kill one or two of the enemy champs. U need armor too cuz vile hurts.
I've gotten bots in 3v3 a lot :( it's the worst. I never have them in 5s tho. One time I was playing with my bf and besides me and him, our third and the entire enemy team were bots. It was as nuts.
Wow I typed a lot. I hope that helps in some way.
GL HF!!!