According to the Spanish artist Francisco Pacheco, there are a number of ways the Trinity could not be represented in art: not as a man with three heads, not as a man with one head and three faces, and definitely not as three men in the womb of the Virgin.
This seems reasonable until one considers the problem Christian missionaries have faced describing the concept of the Triune God. Clearly, a perfectly reasonable heathen’s understanding of the Father, Son, and Holy Spirit might accidentally diverge from accepted doctrine to the “diabolic fiction” described above.
Luckily, Pacheco found it acceptable, even if tacitly, to depict the Trinity as shown in the painting above. This representation was especially convenient for Andean Christians because the Incans had long worshiped Aponinti, Churiinti, and Intiquaoqui, or father and lord Sun, the son Sun, and the brother Son. Not a perfect fit but for a harried missionary bent on stamping out entrenched satanic idolatry you’ve got to make use of all the opportunities that the good Lord provides.
(from Suzanne Stratton-Pruitt’s catalog notes for The Virgin, Saints, and Angels: South American Paintings 1600 – 1825 from the Thoma Collection)