And there will come a time, perhaps, that to the well-known place the lovely animal returns, and tamed, and there where she first saw me that day which is now blessed, she turns her eyes with hope and happiness in search of me, and—ah, the pity— to see me there as dust among the stones, Love will inspire her and she will sigh so sweetly she will win for me some mercy and force open the heavens drying her eyes there with her lovely veil.
If you've had the luck to hear The Police's 1978 hit "Can't Stand Losing You," you'll be familiar with the lines "You'll be sorry when I'm dead/ And all this guilt will be on your head." While Petrarch isn't a teenager committing suicide over his lady-love, he's already figured that the best way into his girl's emotional life is to kick the bucket early. Stanza 3 is the continuation of that fantasy, with an extra bit of wish fulfillment on the side.
Petrarch alludes to a time in the future ("And there will come a time, perhaps") when, inspired by the god of Love himself, Laura will have a change of heart about him (how many love songs have been written about this?). But when she reaches for him? Well, baby, he just won't be there—literally. It doesn't matter that her animal nature has been tamed to his will. She will find only "dust/ among the stones" and will be left with nothing but tears.
But it will be a double-win for Petrarch, because those tears will not only vindicate his love-sickness, but also act as prayers for his soul ("she will sigh [...] and force open the heavens"). It's really the best possible outcome for a star-crossed love like his.
It's interesting to note that, in this fantasy, Laura would return to seek him at the church of St. Claire ("there where she first saw me") and not to the lovely spot by the riverside that opens the poem. In this, Petrarch really shows his cleverness and flair for the dramatic. If she had simply walked up to the river and not found him, she'd be like, "Oh, Petey's not at home today."
But that's not how Petrarch arranges things. In his fantasy, he sends her to the church in search of him so that she will encounter his grave ("see me there as dust/ among the stones"). That's the ultimate guilt trip.
On the technical side, we'd like to mention an interesting translation choice made in line 3. It is translated here by Mark Musa as "the lovely animal returns, and tamed." The line in Italian reads "torni la fera bella et mansueta" and is translated by Robert M. Durling as "the lovely, gentle wild one will return." Musa's translation of the word "fera" as "animal" may seem weird, but there are moments in the Canzoniere when Petrarch assigns Laura's role to an animal (like a deer). Either translation is fair—which one would you go with?
There are also some complicated internal rhyme situations in stanza 3, so make sure you check out the section on "Form and Meter" to stay in the know.