Hello @LEICO24!
There are some steps you need to follow, listed below:
Talk to the Scientist, who will tell you that he needs the seen data of the three legendary dogs.
Find the dogs first in the Bell Tower Basement. To get there, follow the staircase behind the Scientist, and proceed further.
After that, you need to find each dog in its respective location:
Entei - Johto Safari Mountain Zone
Suicune - Tohjo Falls, on the top of the right waterfall.
Raikou - Mt. Silver 3F
NOTE:
A Pokémon with Rock Smash or a Pickaxe is required in order to get Entei's Seen data
A Pokémon with Waterfall is required in order to get Suicune's seen data
After defeating all of them, return to the scientist in Ecruteak City, and pick one of the Bells.
For more information, make sure to check the quest HERE
Let me know if the procedure above worked out for you. I will leave the thread open, in case you have any questions.