How do you get Sceptile Blaziken and Swampert in Pokemon FireRed and LeafGreen?

You can't get any of the Hoenn starters Sceptile (Treecko), Blaziken (Torchic), or Swampert (Mudkip) in FireRed/LeafGreen; you'd need to trade from Ruby/Sapphire/Emerald.

If you do trade a Sceptile, Blaziken, or Swampert (or one of its previous forms) from RSE, you should probably mate that Pokémon with a Ditto in your RSE game first. After you hatch the egg in Ruby/Sapphire/Emerald, you can either trade that baby starter or the original higher-level Pokémon to your FireRed/LeafGreen.

That way, you have one form of the starter in one game and another form of the same starter in the other game.