So Twisted Treeline was suppose to be more focused on small battles and short game time. This has been achieved to an extent.
But people have found it frustrating that some champions are ridiculously better than others in a 3v3 situation rather than a 5v5. Yes, some champions are better at things than others but I believe the root of the problem lies in the slow start of the game.

Twisted Treeline starts players out a level 1 and with the default 475 gold just like in Summoner's Rift. This really limits some players who have champions who cannot be alone and need a support whereas champions who can comfortably solo in SR can just buy one item and be fine. But early dominance in Twisted Treeline is key so that the enemy team has a tough time recovering. So those solo champions will dominate the early game.

What I think would even the playing field out for ALL champions is to have a higher base amount of gold and starting level of at least level 2. Also gaining experience over time would also help speed up the early game.
Now I'm not saying that these numbers should be exactly like Dominion's but adopting the same ideas in Twisted Treeline may make it a more enjoyable experience for all players.

What do you guys think about this?

I would say that no, that shouldn't be. TT and SR share something in common with each other that the Scar doesn't. That's laning, or farming phase of the game. It's important to avoid giving passive exp gain, due to it countering the denial strategy, which is a critical component to the early advantage you are talking about.

Yes, Garen is going to try and deny you because he can dish out harder, and heal better than you. All you have to do is not push the lane. If he is denying you, go gank bot, farm the jungle if you can, or recall. If all else fails, go into the Dragon's den so you can at least keep up in levels. Learning how to accumulate your gold/exp in other ways aside from just farming is important to learn, as it will allow you to adapt to the way someone else is playing. And that is one of the more important things for people to know.