Treecko
Treecko is a Grass-type Pokemon which looks like a gecko. The males of the species are green, while the females are light blue. From birth, Treecko starts knowing two attacks -- Pound, a physical attack which takes hit points from the enemy, and Leer, a status attack which lowers the opponent's Defense stat. When it gains enough experience points in battles to reach Level 16, Treecko evolves into Grovyle, and at Level 36, it becomes Sceptile.
Torchic
Torchic is a Fire-type flying Pokemon which resembles an owl. Male Torchics are predominantly red, while the plumage of the female is mostly yellow. When you first meet Torchic at Level 1, it knows the moves Scratch, a physical attack, and Growl, a status move that lowers the opponent's Attack stat. When it reaches Level 16, Tochic evolves into Combusken, and at Level 36, it becomes Blazkien. Both Combusken and Blazkien are dual Fire/Fighting-type Pokemon.
Mudkip
Mudkip is a Water-type Pokemon which resembles a cross between a dog and a fish. Male Mudkips are blue, while females are purple. At Level 1, Mudkip's attacks are Tackle, a physical attack which makes the opponent lose hit points, and Growl, a status attack which reduces the enemy's Attack stat. Mudkip evolves into Marshtomp when it gains enough experience points to reach Level 16, and then into Swampert when it reaches Level 36. Marshtomp and Swampert are dual Water/Ground-types.
Rival Starters
Your main rival in the game, who will either be called Brendan or May depending on whether you're playing as a boy or a girl, will get whichever of the three Pokemon has a type advantage against yours. So if you pick Treecko, a Grass-type, your rival will have Torchic, as Fire-types are strong against Grass-types. If you choose Torchic, your rival will have Mudkip, as Water-types are strong against Fire. And if you take Mudkip, your rival will have Treecko, as Grass-types are strong against Water.