Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

It has memory's ear
that can hear without
having to hear.

No, in case you were wondering, this is not a riddle. But it is some wrapped-up language. Let's unravel it.
Basically, this is the same instinct idea from the previous lines.
Though memory can't actually hear anything, it can imagine hearing, and that imagining is just as powerful as the sense of hearing itself. And the mind has that power, which Moore tells us in a metaphor.
If you think about it, it's totally true, too. We can hear all kinds of things in our memories. Like right now—imagine the sound of a car alarm. You can literally hear it in your head because you've heard it before (and we bet it's just as annoying in your memory as it is in real life).
So the mind is powerful like memory because it has the instinctive power to do all kinds of stuff. Neat, huh?

Like the gyroscope's fall,
truly unequivocal

A gyroscope is a tool that can spin crazily fast and is usually used to provide stability in navigation devices on boats, for example, where things are really unsteady. They also use them on those crazy Segway scooters, which is how everyone manages to stay upright, despite the fact that they're leaning all over the place.
And unequivocal means "leaving no doubt about it."
These lines address the certainty of the mind, as opposed to the ambiguity or the uncertainty of it. In other words, just as a gyroscope's fall is certain, so is the mind. Oh, and in case you were wondering, that's yet another simile.

because trued by regnant certainty,

If regnant sounds a bit like the word reign to you, well, pat yourself on the back for being on the right track. Regnant is etymologically related to reign, and it means dominant or having a big influence. Like, say, a king or queen.
And trued? Well that's a bit tougher. Usually we think of the word true as an adjective. But here it's being used as a verb. To true something means to align something, to set it right for a particular situation.
So in this case, dominant certainty is righting the mind, so to speak. It's setting the mind up for whatever it needs to do.
Hmm. That whole certainty thing sounds familiar. Yep, it seems to continue to emphasize the "no doubt about it" side of the mind, just as lines 16 and 17 did with that whole gyroscope thing.
Taken together, both lines seem to tell us that even though there are a million forces spinning the mind around, the mind always lands certainly.