Святая Троица. Неизвестный художник, XVIII в.

Про диаграмму понятно, а вот про изображение с тремя лицами растолковывает Википедия:

Even rarer is the depiction of the Trinity as a single anthropoid
figure with three faces (Latin "Vultus Trifrons"), because the Trinity
is defined as three persons in one Godhead, not one Person with three
attributes (this would imply Modalism, which is defined as heresy in
traditional Christian orthodoxy). Such "Cerberus" depictions of the
Trinity as three faces on one head were mainly made among Catholics
during the 15th to 17th centuries, but were condemned after the
Catholic Council of Trent, and again by Pope Urban VIII in 1628, and
many existing images were destroyed.

Цербер в греческой мифологии — трехглавый пес, который охраняет выход из Аида.