how to lock the shared file to be overwritten by other users ---very urgent

Hi experts,

I have giving a shared drive for the 3 users.  The files on the shared drive could be used by 3 of them. They can read, change, and delete them. But the problem comes when 2 of them trying to write on same file . I got to know that after writing the file and saving it , and immediately if other user writes to the same file, it overwrites the whole file . Is there anyway to lock the file when one is trying to write it ?

Thanks,
Sundeep.