[
    // ...whatever else you have set...

    {
        "keys": ["super+alt+6"],
        "command": "set_layout",
        "args":
        {
            "cols": [0.0, 0.33, 0.66, 1.0],
            "rows": [0.0, 0.5, 1.0],
            "cells":
            [
                [0, 0, 1, 1], [1, 0, 2, 1], [2, 0, 3, 1],
                [0, 1, 1, 2], [1, 1, 2, 2], [2, 1, 3, 2]
            ]
        }
    }
]