Instead of breaking it into separate subfigures, create a single mosaic for the whole figure:

import matplotlib.pyplot as plt

def test():
    fig, axes = plt.subplot_mosaic(
        [
            ["A", "A", "A", "A", "A", "A"],
            ["A1", "1", "2", "3", "B1", "C1"],
            ["A2", "4", "5", "6", "B2", "C2"],
            ["A3", "7", "8", "9", "B3", "C3"],
            ["B", "B", "B", "B", "B", "B"],
            ["C", "C", "C", "C", "C", "C"],
        ],
        width_ratios=[1.0, 0.5, 0.5, 0.5, 1.0, 1.0],
        height_ratios=[1.0, 1.0, 1.0, 1.0, 1.0, 1.0],
        figsize=(20, 20),
        layout="constrained",
    )

    for ax_name, ax in axes.items():
        ax.set_title(ax_name)

    fig.savefig('test.png')

test()