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All the starters are great in gen 3.
Sceptile destroys every Water type (of which their are many) with Leaf Blade and it can learn Dragon Claw to rape Drake (and Juan's Kingdra). It has good Special Attack and excellent Speed, so it should outspeed and kill most things.
Swampert has only one weakness (Grass) and that's not that common amongst the main battles you'll face (aside from Winona's Tropius, Sidney's Cacturne and Wallace's Ludicolo), especially since you hardly battle your rival. It's got good Attack and decent bulk, so it should take non-Grass type hits very well which should mitigate its bad speed.
Blaziken has raw power in both stats (which is great pre-gen 4). So while it may not outspeed everything, it will hit everything hard.
