This is how...
First, Go to Mt Silver, and beat Red 3 times. Next, talk to Professor Oak 100 times and save 2 times after that. After that you go to the Goldenrod Radio Tower and talk to everyone in the building. After that, put a Magikarp and a Gyrados in the Daycare until they are both on Lvl 30. Now put your starter pokemon and a ditto in the day care. When you do that, catch all legendaries in the game and put Articuno, Mewtwo, Zapdos, Groudon, Ho-oh and the egg that you got from the man. Then, run around untill the egg hatches. Mewthree should appear after that. If it doesn't work, restart your game and try again.
Now if you did this, I laugh at you. Those who don't believe me, then kudos for you.
First, Go to Mt Silver, and beat Red 3 times. Next, talk to Professor Oak 100 times and save 2 times after that. After that you go to the Goldenrod Radio Tower and talk to everyone in the building. After that, put a Magikarp and a Gyrados in the Daycare until they are both on Lvl 30. Now put your starter pokemon and a ditto in the day care. When you do that, catch all legendaries in the game and put Articuno, Mewtwo, Zapdos, Groudon, Ho-oh and the egg that you got from the man. Then, run around untill the egg hatches. Mewthree should appear after that. If it doesn't work, restart your game and try again.
Now if you did this, I laugh at you. Those who don't believe me, then kudos for you.