In the third stanza of a poison tree what happens when the speaker told his friend that he was angry? What happens when the speaker does not tell...

If I may, I'd like to clarify your question a bit.  The second line of the poem tells the reader that the speaker admitted his anger to his friend.  That wound up being a good thing, because his anger abated and his wrath went away. 

I was angry with my friend; 

I told my wrath, my wrath did end.

Following those two lines in the first stanza, the speaker announces that he was angry again....

If I may, I'd like to clarify your question a bit.  The second line of the poem tells the reader that the speaker admitted his anger to his friend.  That wound up being a good thing, because his anger abated and his wrath went away. 

I was angry with my friend; 

I told my wrath, my wrath did end.

Following those two lines in the first stanza, the speaker announces that he was angry again.  But this time, he doesn't tell his foe.  That winds up being a mistake, because the harbored anger begins to grow. 

I was angry with my foe: 

I told it not, my wrath did grow. 

Stanza two narrates how the speaker nurtures and cares for his anger.  It gets bigger and bigger until it bears fruit in stanza three.  

And it grew both day and night. 

Till it bore an apple bright.

The narrator's enemy sees this fruit and attempts to steal it in stanza four.  That winds up being a mistake for him, because by the end of stanza four the foe is dead, and the narrator is really happy.  

In the morning glad I see; 

My foe outstretched beneath the tree.