Getting Charmander:
Charmander is quite easy to get.Just talk to the guy on the hill after nugget bridge.Answer yes to his question to get Charmander
Getting Bulbasaur:
Bulbasaur is the hardest guy to get out of the 3.Go into the house where you'd normally trade for farfetched in the red/blue version.There'll be a girl with Bulbasaur.If your pikachu is really happy she'll give you bulbasaur.What I did was gave Pikachu a rare candy,a hp up,and the tm mega punch.Then I talked to her.But then I figured out putting him in the daycare makes him happy!Sheesh!I'd try the daycare folks.
Getting Squirtle:
Once you beat Lt. Surge talk to the officer who has Squirtle.You'll get a squirtle!