Stanza 3 Summary

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

Forgive me

Our speaker feels guilty about eating the plums, and straight up asks for forgiveness. Or rather, he commands it, using the imperative mood.
This seems like it's a bit of an easy way out to us—he knew he shouldn't have eaten the plums, and thinks he can get away with just writing a note after the fact.
But, then again, all humans do this. Just look at Adam and Eve, our biblical ancestors—they knew they shouldn't eat the forbidden fruit, but they did it anyway.
Think about a time when you knew you shouldn't do something, but you did it anyway, and had to ask for forgiveness afterwards. This could be something as dramatic as cheating on your girlfriend or boyfriend, or as simple as borrowing someone's pen without asking.

they were delicious

Now the speaker jumps from asking for forgiveness to describing how yummy the plums were.
If the speaker is trying to explain what he did here to help "you" forgive him, we're not sure it's going to work.
He's talking about how delicious the plums are that she won't be able to eat, because he ate them first. Seems like he's rubbing it in a little to us.
This line strikes us as a little funny. Because it's so true to human nature, and it's something we've all said before, it brings a smile to our face.
After all, we all try to justify our misdeeds after we do, them, don't we?

so sweet

Okay, we think, why is he rubbing this in even more?
First, he tells her they were delicious, and then that they were super-sweet? He's kind of digging himself deeper and deeper, right?
Maybe he's just trying to excuse himself by telling her that they were good, but we want to whack the speaker on the head and say, "You're going to make it worse, she can't have any because of you! You should be telling her they were awful!"
But there's something else going on here. For one, the speaker is using the word "so," which is kind of weird in such a minimalist poem.
Let's look at the next line, and see if we can put it all together.

and so cold

Again, we get another line describing what made these plums so good—they're delicious, sweet, and cold.
So the stanza that starts with asking for forgiveness actually ends up being mostly about how yummy these plums were. Maybe it's a hot summer day, which makes the cold plums even that more delicious.
But, like we were saying in our discussion of line 11, it doesn't seem like our speaker would intentionally try to anger the person he's justifying his actions to by describing how good the plums she can't eat were.
Instead, maybe he's just trying to give her some vicarious pleasure. If she can't eat the plums, maybe she'll get some enjoyment out of his note, and be happy that he liked the plums.
Finally, not to be a Negative Nelly, but we have to point out that this is a bit of a non-apology. He's all I'm so sorry I ate your plums. Don't hate me. They were delicious. If we're being honest, there's the possibility that he's not all that sorry at all. Who could be sorry about eating delicious plums? Maybe he's just really worried his wife is going to put him in the doghouse.
Then again, maybe we're just being cynical.