What does M3 stand for in the Clarivoyant challenge?

What does M3 stand for in the Clarivoyant challenge?

During the Systems Jammed mission, destroying the Electrical Fuse Box before the quest tells me, to, completes the challenge. The challenge’s text is:

"Disable the forcefield protecting the second M3 unit before receiving the objective to do so."

But what does "M3" stand for? Is it explained in one of the previous games and I forgot?