About your question:
tplmake more sense compared to the other short 3-letter directory names?”
I don’t think so. “tpl” is not as well-known as “etc”, “src”, “var”, etc. Besides, depending on your font, you may read it as “tp ONE” instead of “tp L”.
I think that “templates” is a perfect name for this. Besides, you never use that in any path when rendering templates, so it doesn’t matter that it’s a bit long.