You don't not unless you trade or cheat
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
You go to professor britch and he gives the fire red starters
All Pokemon from Kanto, Johto, and Hoenn can be obtained in Emerald. Susrskit, Masquerain, Meditite, Medicham, Roselia, Zangoose, and Lunatone must be traded from Ruby or Sapphire. Other Pokemon like all of the starters must also be traded over.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
Cyndaquil and all other pre-fourth generation starters are unavailable in Pokemon pearl and diamond. You must transfer it from Pokemon Emerald. If you do not have Emerald, sign up on a Pokemon trading forum and find someone who is willing to trade you one.
To get all three Starters in Pokemon Emerald you need to trade for them with Pokemon Ruby, Sapphire or Emerald. This is a mechanic used since the creation of Pokemon to encourage players to get together and trade.
Sorry, but the only way to get all the starters from Pokemon Crystal to Fire Red or to Pokemon Emerald is to get a gameshark and do it from there.
you get them by catching all 200 Pokemon
Trade with other people
You can first collect all of the Pokemon for the hoen dex in Pokemon emerald you will get 1 of the 3 Pokemon gold/silver/crystle series starters then use the get all of the starters trick to get them all then migrate them to Pokemon diamond or pearl.
cindaquil must be migrated from emerald version... in emerald, once you catch all the pokemon, the professor offers you one of three pokemon... you may pick from the johto starters (cindaquil, chikorita, and totodile)
To obtain all three starters you need to trade with someone.
You can only get one of the three Starters in Pokémon Emerald without use of cheating or trading. You can trade or use an action replay code to get all three (I don't know the code, so I can't help you there). Hope this helped.
you need to catch or see all the Pokemon in the national dex.
Yes you have to trade because you can't get all the Pokemon alone.