Mitsutorii is a combination of three myojin-gate torii (a pillared gate-like entrance to a Shinto shrine), and it is sometimes called yotsuashi torii (four-legged torii) because it has four pillars.
Although the Mitsutorii in Omiwa-jinja Shrine cannot be actually seen because it is in the prohibited area, the official website of Omiwa-jinja Shrine has the picture of the model of Mitsutorii in 'Precincts Guide - Mitsutorii.'
The official website of Omiwa-jinja Shrine doesn't mention the period when the Mitsutorii was formed or its derivation.
There is Mitsutorii in front of the haiden (a hall of worship) of Mitsumine-jinja Shrine.
It is also unclear about the derivation of Mitsutorii of Mitsumine-jinja Shrine.