Stanza 3 Summary

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

In the golden lightning

Here's a little more drama and passion. Shelley really goes for broke in this poem with the gorgeous imagery.
"Golden lightning" refers to a sunset here, but it also makes us think of crackling electricity and intense, almost scary power.
In just over ten lines, our speaker has turned a chirpy little bird into some kind of lightning-riding fire spirit.
It's kinda metal, actually. Think we could grow our hair out and start a band called Skÿlark? Maybe "Golden Lightning" would be better? Okay, so maybe that's a terrible idea.

Of the sunken sun

Aha! So, this is where that golden lightning is coming from: a beautiful sunset lighting up the sky.
Also, we get some pretty high-quality alliteration here ("sunken sun").

O'er which clouds are bright'ning,

We think this moment is all about losing yourself in the brightness and beauty of the sky. In the literal sense, the speaker is describing how the sun, when it sets, can fall below the clouds and brighten them from below.
Such a beautiful image, though, is also meant to stir the emotions. All that gold and fire and light kind of washes over you, until you don't feel anything but the joy and excitement of the skylark's song and its flight.

Thou dost float and run;

If the last line was all about feeling the beauty and joy of light and color, this one is about the freedom and pleasure of movement.
The skylark is pure freedom, moving without any effort. Its flight is like floating—pure and easy.

Like an unbodied joy whose race is just begun.

The skylark is so free, in fact, that it's as if it doesn't have a body. That connects to the idea that it's more "Spirit" than bird (see lines 1-2).
The bird is pure joy, full of the possibility of a new beginning. It's not tired and used up. It's full of energy and freshness, as if "its race is just begun."
You might have noticed by now that the last lines of each stanza are longer than all the others, with a total of six beats rather than two. Their rhythm is different, too. While most lines in the poem are written in a rhythmic pattern called "trochaic trimeter" (which is a fancy way of saying that it sounds like: DA-dum, DA-dum, DA-dum), this line, and all the other last lines of each stanza, is written in what's called iambic meter. ("Like an unbodied joy whose race is just begun.")
You can cruise on over to the "Form and Meter" section for more about all that.