Re: 3v3 best picks
Weaver would probably be a top pick in 3v3. He owns a 1v1 lane and he owns late game hard. Also he's a lot harder to kill with only 3 players.
I'm not very sure about the rest. There are so many good choices that come in mind after a bit of thinking: Omniknight, Shadow Demon, Doom, Furion, Windrunner, Broodmother, Shadow Fiend, Balanar. Then there are the heroes usually not seen in serious games that get stronger the fewer players there are in the game: Slark, Slardar, Bane, Barathrum, Meepo, Bristleback, Huskar, etc.
I guess there's no use theorycrafting it, you are just going to have to play 3v3 and find out the best strategies yourself. I can only give you a list of suggestions.