Stanza 3 Summary

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

And death shall have no dominion.
No more may gulls cry at their ears
Or waves break loud on the seashores;

As usual, our final stanza starts with the same refrain as the prior two stanzas. By now we can be so bold as to assume Thomas was aiming to keep things highly organized, Romantic-poet-inspired, but also a bit modern with those slant rhymes.
Line 20 seems to be moving away from all the tortuous stuff of the second stanza. Here the speaker is looking more generally at nature with its "gulls" and "seashores." Sounds almost serene.
Notice, too, that the speaker is getting a bit nostalgic here in terms of sound. The imagery here is more aural (sound based) than visual. And the "no more" clause adds to that romantic nostalgia.
If we were to break the stanzas down into different kinds of imagery and sense perception, we'd say the first is cosmic/visual, the second is earthly/sensual (in a tortuous way), and here the third is also earthly but in an aural way.
Line 21 emphasizes the aural quality of the third stanza by including the idea of waves breaking "loud." So by now we kind of feel like Thomas has covered all the earthly, cosmic, and spiritual bases. Phew.

Where blew a flower may a flower no more
Lift its head to the blows of the rain;

Here we have some more earthly images, this time with pretty flowers. We still have a bit of the aural imagery happening too with the blowing of the wind. It's all sticking together rather nicely, don't you think?
But here's a question: why all the non-human images? What happened to all the dead people? We want dead people… er, not.
Okay, maybe we don't want dead people, but still we're noticing a far more serene look at the world. It's less burdened by people problems. It's more pastoral (think green pastures).
And that kind of makes sense since we're nearing the end of the poem. It's no use fighting against nature and death anymore.
We surrender. We might as well get some serene imagery to go with.
And we surrender with that refrain in mind. We know those pretty flowers won't be blowing in the wind anymore, but we also know there's something beyond the physical world in this poem.
Likewise, line 23 is getting at the beauty and hardship of the natural world that will be "no more" after death. That poor flower won't have to "lift its head" to the "blows of the rain" anymore.
It doesn't take much to connect the symbolism of line 23 to the more human experience of enduring "blows" in life. So we, like the flowers, will no longer get knocked down by all the painful stuff.

Though they be mad and dead as nails,
Heads of the characters hammer through daisies;

Ah, there are those dead people again. Thomas must have read our minds. So here in the third stanza he's blending it all together again: dead people, pretty flowers, and death. Remember, unity is a big motif in this poem.
And the dead folks he's choosing to end with are the "mad" ones again. Poets kind of have a thing for crazy people.
Line 24 isn't making any apologies for those "mad and dead as nails" folks. So the speaker is again emphasizing the full extent of how far people can "rise up."
And you're probably getting excited because we have a simile in line 24, too: "dead as nails." Maybe you've heard that expression, maybe not, but it's a common idiom too. Put that one in a jar for safekeeping.
Line 25 has some figurative language that looks to be physically "hammer[ing]" through that supposed dominion of death. Those "heads" aren't going to let anything stop them from rising up.
There's a euphemism there, too, which plays with the "pushing daisies" expression. It's an old fashioned little saying that charmingly refers to people being buried beneath the ground and pushing out the daisies that have been growing over them.

Break in the sun till the sun breaks down,
And death shall have no dominion.

Line 26 is another one of those tough Thomas eggs to crack. All the figurative language of "break[ing] in [the] sun" makes us wonder if it's the dead people causing all the cosmic raucous.
But the point seems to be that all those hammering heads of characters are quite powerful, even in a cosmic sense. After all, breaking in the sun certainly wouldn't prove to be an easy feat.
So the rising, and breaking of the sun, will persist until the "sun breaks down." Presumably the speaker is referring to the end of days for everyone. The sun can't burn forever, as we know.
And even after that sun breaks down and the end of days is here, "death shall have no dominion." A little judgment day won't stop tough people like us Shmoopers, right?
All in all, we've swept the universe in a rather short poem while bearing in mind that death is not the end. So the next time you feel as if studying for that calculus exam might just kill you, think of Dylan Thomas's famous poem for some inspiration.