How to compare contents of two folders against third one?

["Alexander V. Makartsev" <avbetev@gmail.com>]

    Hello everyone.
    
    I have a "/source-folder/" which contains very large tree of folders
    and files.
    I've manually copied a set of folders and files from it to a
    "/destination-folder-one/" and
    copied another set of folders and files to a
    "/destination-folder-two/".
    
    Now, is there an effective way to compare combined contents of two
    folders "/destination-folder-one/" and
    "/destination-folder-two/" against a "/source-folder/" to show if
    there is anything that was left out?
    
    For now I've tried "diff" and "rsync" to accomplish this.
    But diff apparently can't compare combined contents of two folders
    with another folder.
> $ diff -r
>         /destination-folder-one/ /destination-folder-two/
>         /source-folder/
>         diff: extra operand '/source-folder/'
>         diff: Try 'diff --help' for more information.
>     
    
    And rsync scans folders and files that are already exist inside both
    destination folders and "/source-folder/" and outputs nothing.
> $ rsync -r --size-only --dry-run
>         /destination-folder-one/ /destination-folder-two/
>         /source-folder/
>         $
>     
    
    I could go on a wild chase to "ls" contents of both destination
    folders, concatenate the results, sort them somehow, 
    do the same to a source folder and compare the resulting list files.
    Also create test cases to check if results are reliable.
    But before I do that, is there a better way to accomplish the task?
    Maybe some parameter for diff or rsync that I missed or another
    utility?

--
With kindest regards, Alexander.

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