Stanza 3 Summary

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

And on that cheek, and o'er that brow,
So soft, so calm, yet eloquent,
The smiles that win, the tints that glow,

The woman's smiles and her healthy blushes ("tints") that "glow" on her "cheek" and "brow" are serene and "calm." ("Brow" is just a poetic way of saying forehead.)
In other words, she's quiet and rather elegant – she doesn't joke and laugh a lot; she seems to be more of the lovely and regal type.
But even though she's quiet and "calm," her "smiles" and blushes are "eloquent" Her face is very expressive, even if she doesn't say much out loud.

But tell of days in goodness spent,
A mind at peace with all below,
A heart whose love is innocent!

But what, exactly, do those "smiles" express? We're so glad you asked: Byron tells us that they reflect all the time that the woman has spent doing good deeds.
She's certainly not just a pretty face – she's also kind and good, which is why she's able to look so "calm" and serene: her conscience is at rest.
The woman's serenity and "smiles" also reflect the calmness of her mind. Because she's a good person, her "mind" is at "peace with all below" (everyone on earth).
Not only that, but her "love is innocent." This could mean that she's not in love with anyone, or it could mean that she is, but that her love is pure and "innocent" – in other words, that it's not a sexual love.