Hello,
I would recommend one of following approaches:
1) You can simply divide those images to three subsection for each museum:
-Museum 1
--Events
---Image
---Image
--Exhibitions
---Image
---Image
--News
---Image
---Image
-Museum 2
--Events
---Image
---Image
--Exhibitions
---Image
---Image
--News
---Image
---Image
-Museum 3
--Events
---Image
---Image
--Exhibitions
---Image
---Image
--News
---Image
---Image
2) You can add an extra field (dropdown or radio buttons) to the file type where you can choose where the image belongs to
Best regards,
Jan Hermann