Elexarpeh Comananu Tabitom

[Fio Praeter Humanus]

In the Portal ritual of the Golden Dawn you will find the following short invocation:

"Ol Sonuf Va-orsagi Goho IADA Balata. Elexarpeh Comananu Tabitom. Zodakara, Eka Zodakare Od Zodameranu. Odo Kikle Qaa Piape Piamoel Od Vaoan."

It also shows up in some versions of the Opening By Watchtower. It obviously is an invocation of Elexarpeh Comananu Tabitom
Which are the governors of the 10th aether. Regardie described them as being the archangels set over the tablet of union, which I assume this idea comes from that their sigils are on the black cross.

My question is who exactly are these three and what is their purpose? Why did the GD choose this invocation to awaken the tablet of union rather than the 1st and 2ed key?