The respondent below did not understand the context of Bulbapedia's information correctly. Bulbapedia states that:
"Each of these moves can be taught only once."
By that it means that the move tutor can only teach Charizard, Blastoise, and Venusaur the moves once, but does not refer to only being able to teach one Pokemon out of the three.
If you have traded in the past in order to get the two other starter Pokemon that you were not able to catch normally in Fire Red and Leaf Green and want to learn Frenzy Plant, Blast Burn, and Hydro Cannon. What you do is make sure that all of the starter Pokemon are fully evolved and have maximum friendship towards you.
Then you travel to Boon Island (Second Sevii Island) and you walk across the island and up the stairs until you reach the house with the old lady who is a move tutor (Cape Brink).
Before you enter the house you put one of the starter Pokemon (Charizard, Blastoise, or Venusaur) in the lead (front of your Pokemon group/party). Enter the house and speak to the woman where she will ask if you would like to learn her move, but will only be able to teach it to the Pokemon once. You agree to learn the move, have the Pokemon learn it, and then exit the house.
Once you are outside the house swap the lead Pokemon who already learned the move with another starter who has not. You enter back into the house and talk to the move tutor. She will ask again to see your lead starter to learn the new move.
Once she has thought all of the starters new moves she will say that her work is done, and you will be left with three starter Pokemon with moves up to 150 attack plus STAB bonus.
I hope this helped anyone who was having trouble!
