In this video, you can see how to obtain all 3 starter Pokémon in Pokémon Blue Version. Here's a brief step-by-step guide:
1. Start the game as usual, until you've chosen your 1st starter.
2. Save the game and reset.
3. Now you'll see a "continue" and a "new game" option. Choose "new game".
4. After selecting your name, etc., save the game.
5. Turn off your GameBoy directly while saving! The timing of turning off the GameBoy is crucial and determines the outcome! If it was too early, you'll end up with corrupted save data. If it was too late, the save will be overwritten.
6. If the timing was right, you'll have a starter in your party and can potentially get another starter. You can use this glitch multiple times to obtain all 3. Good luck.