Stanza 3 Summary

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

From School-boy's tongue no Rhet'ric we expect,
Nor yet a sweet Consort from broken strings,

In the beginning of this stanza, Bradstreet gives us a few metaphors for her inferior skills as a poet. The basic idea in all of these comparisons is that you shouldn't expect something or someone to do more than it was made to do, so why should you expect her, a lowly woman, to write like a famous man?
For example, a little kid ("School-boy") wouldn't be expected to give fancy speeches ("Rhet'ric"). You also wouldn't expect beautiful melodies ("sweet Consort") from a musical instrument with "broken strings." Both of these things are analogies for her poetry.

Nor perfect beauty where's a main defect.

You also wouldn't expect something or someone to be perfectly beautiful if you saw some major flaw ("main defect").
Essentially, Bradstreet's speaker is telling us to keep our expectations reasonable (i.e., low). Just like a broken instrument or flawed beauty, there's apparently only so much you can expect from a woman poet. Again, physical beauty is an analogy for skill as a poet.
Man, we just want to give her a pep talk, don't you?

My foolish, broken, blemished Muse so sings,

In this line, the poem picks up the idea of the "main defect" from the line before and applies it directly to the speaker's own poetry.
Since Bradstreet didn't believe that the Greek gods were real, you can think of this "muse" as a personification of artistic inspiration. The speaker is giving human qualities (like foolishness, or the ability to sing) to an idea.
You might imagine that a poet's muse would be beautiful, perfect, appropriately godlike. Bradstreet's speaker wants us to know that's not the case with her muse. Apparently her muse is flawed in all kinds of ways, and she sings like it too.
We may say this a bunch, but what makes this poem so tricky (and so cool) is the difference between what Bradstreet says and how she says it. While the line means "my poetry kinda stinks," the line itself is great. It's not broken or blemished at all, in fact it's vivid, well-balanced and in perfect iambic pentameter. (Check out "Form and Meter" if you don't believe us.) So while she tells us she's lousy, she's also showing us that she's anything but. It's sort of a great way to get back at the haters while pretending to agree with them, isn't it?

And this to mend, alas, no Art is able,
'Cause Nature made it so irreparable.

There's nothing she can do to fix the flaws and brokenness that she sees in her poems. Because the speaker's muse is broken and foolish, no amount of careful work ("Art") can fix the poetry she inspires.
This is Nature's fault—it created this irreparable flaw, and the speaker can't fix it. What's that flaw, exactly? Well the speaker doesn't come out and say it, but it seems to us that the flaw is, sadly, that she was born a woman. (We know, we know—but keep in mind that this was way back in the seventeenth century, folks.)
We also want to point out how active Nature is here, with all its making and doing. It's almost like it's a character in the poem.
For that matter, Art, even though it's "unable," is treated like a character too. When a poet gives human qualities to a thing or an idea like "Nature" or "Art," that's called personification.
As we've said before, it's not totally clear to us that Bradstreet completely buys this line about female inferiority, but that's what her speaker is going with for now.