OK, thank you for the screenshots, they help a lot.
The underlying problem is that the ST3 and MP2 Sounds are in separate folders, as shown in
https://imgur.com/8AowvEK. Only one high-level folder can be selected in ST3 at a time, so you see only the ST3 (and Imported sounds) OR MP2. NOrammly the MP2 sounds installers put the MP2 instruments and sample files into sub-folders of the ST2 "Instruments" and "Samples" folders.
I am hesitant to post a solution as there are too many unknowns. Can you let us know:
(a) what is in that "Suoni and 1/IK Multimedia" folder?
(b) what is in that "Suoni and 1/Sounds/SAMPLETANK" folder?
(c) what is in that "Suoni and 1/Sounds/SAMPLEMOOG" folder?
(d) what folder is the SampleTron library in?
(e) do you want to use ST3 and MP2 only or will you still need to use SampleTron or SampleMoog?
(f)
https://imgur.com/3xTtwji suggests that the MP2 library has been IMPORTED into ST3. Did you do that?
(g) would you be prepared to delete the libraries and reinstall them?