If you start Meld from the command line, you can tell it what to do when it starts.

For a two- or three-way file comparison, start Meld with meld file1 file2 or meld file1 file2 file3 respectively.

For a two- or three-way directory comparison, start Meld with meld dir1 dir2 or meld dir1 dir2 dir3.

You can start a version control comparison by just giving a single argument; if that file or directory is managed by a recognized version control system, it will start a version control comparison on that argument. For example, meld . would start a version control view of the current directory.