flatten three level deep tree

[et_pb_section admin_label="section"][et_pb_row admin_label="row"][et_pb_column type="4_4"][et_pb_text admin_label="Text" saved_tabs="all" background_layout="light" text_orientation="left" text_text_color="#000000" use_border_color="off" border_color="#ffffff" border_style="solid" global_module="1167"] In the previous examples, we saw how to filter two level deep trees with map, filter and concatAll. Let's try three. ---
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