Stanza 3 Summary

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

Do they matter?—those dreams from the pit?

The speaker shifts gears; he now asks if "they matter," the "dreams from the pit"? Hmmm. We're gonna go out on a limb here and say that yes, they do.
Even though he's not talking about missing limbs anymore, the speaker still manages to evoke depression, sadness, and the like.
"Pit" probably refers to the trenches in which soldiers lived and from which they launched their attacks during World War I.
And the "dreams from the pit" probably refers to the young soldier's hopes and aspirations for after the war.
It's a little tricky to read the tone of "do they matter" here. The speaker suggests, on the one hand, that the soldier's "dreams" have been destroyed as a result of the war, and thus no longer matter.
The speaker also, however, assumes the same sarcastic tone he's maintained throughout; he acts as though the loss of those "dreams" is no big deal (just like the loss of sight and legs), for reasons he explains in the following lines.

You can drink and forget and be glad,
And people won't say that you're mad;

According to our speaker, the destruction of the soldier's dreams is no big deal because he can "drink and forget and be glad."
And if he gets wasted and forgets about the war and acts happy, nobody will claim that he's (the soldier) insane. Excellent?
Yep—we were being sarcastic. And so is our speaker. It's as if he's saying, "who cares about your dreams boy? You can just get drunk and forget about them after the war, and everything will be hunky dory."
Notice the rhyme on "glad" and "mad" here. Sassoon's doing some fancy footwork here, connecting those two words via sound. it forces you to consider what gladness has to do with madness. And in this case, it's everything.
For the speaker to be glad, he'd have to be a little mad. After all, he's in a right sorry state, and we imagine he'd have a pretty hard time finding the sunny side of the street.

For they'll know you've fought for your country
And no one will worry a bit.

Bring it home, Siegfried.
Here, the speaker assures the soldier that other people won't call the soldier cray-cray because they will understand that he's fought for his country; nobody will worry at all about him or his well-being.
We have to say, these concluding lines are totally sarcastic; the speaker is acting like fighting for one's country is a good thing, and that people will respect it, but he's mocking the entire idea.
According to the poem, after all, fighting for one's country destroys one's limbs, eyesight, and dreams, and someone really should be worrying about this guy.