This depiction of the Trinity is a manuscript illustration. It shows a knight with a Trinity shield going forth to fight against the Seven Deadly Sins. Here is a closeup of the shield:
The manuscript it illustrates is the 13th century Summa Vitiorum (Treatise on the Vices) by William Peraldus. The artist is unknown. The manuscript is located in the British Library (not the same as the British Museum).
Here is a view showing the Seven Deadly Sins:
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Here is a depiction of the Trinity that is very different from those relying on Genesis 18. It is by El Greco. It was painted in the late 16th century and is presently in the Prado museum in Madrid, Spain. The medium is oil on canvas and it measures 118 inches by 70 inches.
There is an interesting Wikipedia article about El Greco who was indeed from Greece (Crete), spent time in various cities and eventually settled in Spain where he received important commissions, one of which included this artwork.
Here is the well-known 15th century Trinity icon by Russian artist and monk Andrei Rublev. Like the Ravenna Mosiac, it is based on the Hospitality of Abraham as described in Genesis 18. The medium is tempera on board. It is large - 56 inches by 45 inches.
At the link above, one can find a very large amount of information about the icon and its many journeys and treatments over the years as well as information about Rublev himself and similar artworks. There is no evidence that Rublev ever saw the Ravenna Mosaic. Rather, both artworks are in the tradition of classical Byzantine iconography.
One interesting facet of the story of the icon is that in 1599-1600 Tsar Boris Godunov had the icon covered with a gold riza (protective plate) that remained on the icon for centuries. It had been covered before but he provided an updated covering.
