Stanza 3 Summary

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

Certainty, fidelity
On the stroke of midnight pass
Like vibrations of a bell,

Our speaker is once again sharing his pessimistic point of view. Here he says that certainty and fidelity (or faithfulness) don't last (kind of like how he said in the first stanza that the beauty of childhood doesn't last).
He brings in two very obvious symbols of time here: a clock and a bell, both which strike and let us know that time is moving on. It might seem a little heavy-handed: does he need to reference both a clock and a bell? Wouldn't one symbol of time be enough? Apparently not. He really emphasizes his point: nothing lasts forever.

And fashionable madmen raise
Their pedantic boring cry:
Every farthing of the cost,
All the dreaded cards foretell,
Shall be paid [...]

Once again the speaker brings in some strange characters. Who are these fashionable madmen? Why are they yelling pedantic (or obnoxious and nit-picky) things?
These madmen are much like the hermit of the previous stanza. They're part of the speaker's imagination.
And what are they yelling? Well, the colon in Line 25 acts almost as a quotation mark: what comes after it is a description of the madmen's words. Basically, they're yelling about money and tarot cards.
Still confused? Don't worry. These are tricky lines. A farthing was a British coin (it's no longer in use, so don't try to exchange your dollars for farthings next time you're in the UK). The "dreaded cards" that "foretell" are most likely tarot cards, which are supposed to predict the future.
So, what the men are really yelling about is the future. They're saying that we have to "pay up" before the end. We have to do what the tarot cards predict; we don't have any choices in life. After all, the bells are tolling – and they're tolling for us.

[…] but from this night
Not a whisper, not a thought,
Not a kiss nor look be lost.

These lines are starkly different from the ones that come before. The earlier lines of the madness were loud: clicking tocks, tolling bells, madmen crying in the streets. These lines are quiet and intimate instead.
It doesn't matter what's going on outside to the speaker; all he wants is to remember the night that he's had with his beloved.
The repetitive and parallel phrases that begin with "not" all emphasize this point. He really, really, wants to remember everything. It doesn't matter how quickly time flies and attempts to erase the past. The speaker is going to hold onto every last detail.