ubuntu with samba share - user access right problem

i m using uBuntu as file server, i using samba share. i got 3 user here which is user A, user B, user C.

At the same time in my uBuntu side i have created 4 folder with access right which is user A can access to folder A, user B can access to folder B and user C can access to folder C, while folder D is public which mean everyone can login and read and write file n folder. Everything is correct with own user can access to own folder.

Problem I facing is user A copy a file to folder D which is public folder to share out files, user B and user C can read the file from folder D but they cant delete or edit the file, they just can read. It happen to other user as well. Files copy from each user can only be read and write by the same user, while the others just can view it.

What I need to do to make it like this==> every file copy from any user as long as it is in folder D then everyone can read and edit it.

How to do this?

Thx for sharing n help

best regards