Stanza 3 Summary

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

Cruel and sudden, hast thou since
Purpled thy nail in blood of innocence?

Oh no! She killed it! Murderer! Suicide! Blaspheme!
Wait, the sky didn't fall, after all. She killed the bug, but all that happened is that she stained her fingernail with the "purple" blood.
Nonetheless, the speaker is crestfallen. He calls her action "cruel" and hasty.
She has taken the blood of an innocent, which also has vaguely religious overtones, alluding perhaps to the death of Jesus Christ.
Still, though he sounds shocked, we know that nothing too terrible has really happened.

Wherein could this flea guilty be,
Except in that drop which it suck'd from thee?

He expands on his notion of the flea's innocence. It must be because he's so sympathetic to insects...right.
He asks how the flea could have been guilty of anything except taking one small, teensy drop of her blood.
We can imagine her response: "Yeah, well, I'm going to be scratching that bite for days!" We don't blame you, lady.

Yet thou triumph'st, and say'st that thou
Find'st not thyself nor me the weaker now.

The woman has "triumphed" over the flea, and she believes she has also triumphed over the speaker's argument.
Let's recap that argument, shall we? The speaker had said that because the flea contained both his and her blood, killing it would be like killing both of them.
But now that she has actually gone through with the violent deed, she finds that she hasn't lost any of her strength. She feels exactly the same. No calamity has befallen them.
Doesn't this prove that his argument was just a load of hogwash?

'Tis true; then learn how false fears be;
Just so much honour, when thou yield'st to me,
Will waste, as this flea's death took life from thee.

Ah, but never underestimate the tenaciousness of the speaker in a Donne poem. He's always two steps ahead of you.
In this case, he readily admits that his earlier argument has fallen apart. He brushes aside her objection by simply conceding, "'Tis true."
Then he brings out his ace card. The whole argument has been a way to prove that having sex with him wouldn't be such a disastrous, sinful, shameful thing. He will now prove that her earlier "fears" have been unfounded.
When she "yields" to his seduction, she'll discover that the amount of honor she loses will be equal to the amount of life she lost when she killed the flea.
Which is to say: Nada. Zippo. Zilch. None.
Just as she felt no less powerful after killing the flea, she will be no less of a maiden after she has slept with the speaker.
And, not surprisingly, the poem ends before she can even respond.