The starters are much balanced in Ru/Sa IMO.
Good against the first gym, probably the worst from the three.
The most useful from the three to have in your party. this bearing in mind that Fire types are rare in this game. Blaziken its final form is one of the strongest fire type in the game. Plus, Blaziken would be useful for elite four.
Quite helpful against the gym. No particular weakness in general against them, and Swampert has a slightly better stats than Blaziken and Sceptile.