When Shall We Three Meet Again?

As you can see in the pictures above and below, this phrase "When Will We Three Meet Again?" or "When Will We 3 Meet Again?" was some kind of meme from about 1890 to 1910.

The reference is to the first lines of Shakespeare's "Macbeth" (i.e., Act 1, Sc. 1, ll.1–5), spoken by the three witches (full scene at the end of this post):

"When shall we three meet again?
In thunder, lightning, or in rain?"
"When the hurlyburly's done,
When the battle's lost and won."
"That will be ere the set of sun."

Presumably the answer to this quote-question is "soon" or "when the job is done" or "after the battle" (for a soldier I guess).

But what has me puzzled is why is there always a donkey/mule/ass in the picture? Any suggestions?

(And FWIIW, I am not the only person puzzled!)

UPDATES AT THE END OF THIS POST

* * * * *

{Thunder and lightning. Enter three WITCHES.}

First Witch
  When shall we three meet again?
  In thunder, lightning, or in rain?

Second Witch
  When the hurlyburly's done,
  When the battle's lost and won.

Third Witch
  That will be ere the set of sun.

First Witch
  Where the place?

Second Witch
  Upon the heath.

Third Witch
  There to meet with Macbeth.

First Witch
  I come, Graymalkin!

Second Witch
  Paddock calls.

Third Witch
  Anon.

All
  Fair is foul, and foul is fair:
  Hover through the fog and filthy air.

{Exeunt.}

* * * * *

UPDATES!
A colleague at Monash writes:

"I think it's a comic insult: the general form seems to be that two donkeys invite the reader to join them, thus implying that the reader is also a donkey. As a catchphrase it might have originated in one of the many travesties of Macbeth that were so unaccountably popular in the C19."

Terry L. Meyers writes:

“The third donkey, as your collegue suggests, is surely the reader—and he has found an excellent example of a postcard which has a reflective panel in it, which makes this very clear.

Terry has also found a reference to this meme from 1868 in a journal/annual called The Child’s Friend

If we look into a photograph shop window we are sure to see somewhere a picture of two poor donkeys, and underneath we read the startling question, ‘When shall we three meet again?’ What a take-in, is it not? What does it mean—that we are to be ashamed of being like a donkey? Why do people say ‘What a donkey you are!’ when you have done some very stupid thing? Because donkeys are supposed to be stupid. I will tell you a little about our four-footed friends, and then you can judge for yourself about poor Neddy and his vices.

[…]

Let us remember that to give poor Neddy kind words and good treatment is to help him up in the world, and that the sooner he is helped up the sooner we shall cease to think of him as all that is stubborn and stupid, and the sooner we shall lose the reason for minding the old saying—‘When shall we three meet again?’
[Cousin Louise, “The Donkey,” The Child’s Friend 4 (1868): 41–42.]

Of course, this doesn't really answer the question of why the question asked by Shakespeare's first witches involves donkeys, but if this meme was "old" in 1868 it does suggest the answer lies earlier in the nineteenth century than I thought.

MORE UPDATES! Clare G. and Ken R. have come to my rescue. Both point out that there is a reference to the "We Three" joke in Shakespeare's Twelfth Night. So, a lot earlier than the nineteenth century! As Clare explained, there is a reference to it Brewer's Dictionary of Phrase & Fable:

We three. "Did you never see the picture of We Three?" asks Sir Andrew Aguecheek (Shakespeare: Twelfth Night, II, iii) – not meaning himself, Sir Toby Belch, and the clown, but referring to a public-house sign of Two Loggerheads with the inscription, 'We three loggerheads be," the third being the spectator.

I don't know why I didn't go straight to Brewer's. The internet has made me lazy, Brewer's Dictionary was my first stop for anything like this for years: I have three editions, spanning a century of revisions and alterations, all very well-worn! But they are at work, of course, and it is a long weekend, and, like I said, the internet has made me lazy. But the answer he gives pretty-well nails it.

Now that I know the meme exists in the Shakespeare cannon there should be no difficulty chasing up the folk and literary traditions. Everything in fact. I'll just go straight to the latest Arden edition of Twelfth Night.

* * * * *

But the thing that still puzzles me is the conflation of Macbeth and Twelfth Night. It is a witch who asks "When Will We Three Meet Again?" A witch talking to two other witches. No donkeys in the entire play. No suggestion that the other two are donkeys, donkey-like or donkey-headed (like Bottom in A Mid-Summer Night's Dream). So, what gives?

Perhaps the conflation can be explained only by the appropriateness of the question (unrelated in any way to context of Macbeth) to the "we three" meme. The fact that the quote has been detached from its context in this way is kind-of interesting. Lots of Shakespeare quotes have been detached in this way.

I can't help wondering if it possible to identify the point at which the detaching and conflation occurred, but I don't really have the time to try to track it down, so it looks like this will remain one of life's little mysteries!