I don't recall an entire essay on the trident. Trapezoids obviously, but not the trident. He does mention the trident briefly in the Satan Speaks! essay, "The Third Side". But as for an entire essay, it might have been one of his otherwise unpublished essays from something like The Cloven Hoof. I didn't know the grotto that published "The Trident" had named it after one of Dr. LaVey's essays, though.
I'm not certain of the publications origins, as far what it was named after. I'm just reporting the information I was giving, according to this person's memory. The grotto that published "The Trident" may not have named it after one of Dr. LaVey's essays.
But, you're right, it could well have been in one of the Church of Satan
's magazines like The Cloven Hoof
Who knows. I just relate this information back to the asker.