Stanza 3 Summary

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

That sphinx-like face, remote, austere,

Now the Old Year has turned around and we can see his face. The speaker describes it as "sphinx-like."
The sphinx was a monster in Greek myth that asked riddles of people. That image of a scary, mysterious creature has turned into a word for anyone who appears to be secretive or hard to read.
Apparently this face is also "remote" (far away) and "austere" (strict and cold, without a lot of feeling). Not exactly warm and fuzzy.

Let us all read, whate'er the cost:

The speaker invites the "audience" (the people, like him, who have lived through this year) to read the face of the Old Year.
This is kind of odd, since all the descriptions in the line above make it seem like this face would be pretty hard to read. The speaker is building up the sense that looking back on the year that just passed is difficult, and might even be a little bit dangerous, or painful, that there might be a "cost."

O Maiden! why that bitter tear?

This part might seem confusing at first. Where the heck did this "Maiden" come from?
Just remember the theater metaphor that is tying this whole thing together. The speaker is imagining that he's in an audience. Until now, he's been looking up at the stage, where the Old Year is standing, but now he's looking at the people in the audience who surround him.
Apparently the speaker sees a young woman crying near him, and he asks her why.

Is it for dear one you have lost?
Is it for fond illusion gone?
For trusted lover proved untrue?

The Maiden doesn't say anything, but the speaker imagines what might have happened in the past year to make a young woman cry. He guesses that maybe a loved one died. Or maybe a "fond illusion" (a silly dream or fantasy) has disappeared. Or maybe her boyfriend has turned out to be a cheater.
Be sure to notice how downbeat most of this poem is so far. Not depressing exactly, but sort of blue and melancholy.

O sweet girl-face, so sad, so wan
What hath the Old Year meant to you?

Here our speaker finishes up his thoughts about the Maiden, with her "sad" and "wan" (that means pale) appearance. He still doesn't know what happened to her, but we can be pretty sure that it's been a rough year.
Check out how he creates a contrast between the "sweet girl face" and the "sad" expression. That kind of balance between young and old, happy and sad is a major theme throughout this poem.