The ascription of "mystical" cannot be arbitrary -- it is the context that decides whether it is warranted, as in the following use of "three":

"Here is the man with three staves".

--- On Tue, 4/20/10, Nancy Gish <[log in to unmask]> wrote:

According to Eliot, it is the Shackleton expedition and the Journey to Emmaus, both of which had examples of this phenomenon.  It is three, no doubt, because in the Journey to Emmaus it was three.  I do not think Eliot shows any tendency to numerology anywhere, unless you want to note that he writes in movements like music.
There are many explanations for this phenomenon of another being experienced as present who is not literally there.  For example, it is a kind of hallucination frequent in soldiers and others who watched trauma during WWI.  The masses on the street, for example, echoes Bertrand Russell's image, and Stetson echoes many, many accounts of soldiers who think they see someone even though the person died in the War.  Another literary example is the hallucinatory scene in Mrs. Dalloway of the former soldier's dead officer, imagined as walking toward him.
My point is that there is no single, absolute way to define this as somehow mystical--though that may be one experience of it.  And if every time we encountered three of anything it meant a mystical experience, we are all having them all the time.  One may, of course, read it that way if one finds it useful, but it is not a fact or an unquestionable meaning.

>>> Chokh Raj 04/20/10 10:27 AM >>>

Significantly, there is the triple configuration of the line:
 "Who is the third*who walks always beside you?"
"There is always another one walking beside you".
"—But who is that on the other side of you?"
The triple emphasis here appears purposive, especially vis-a-vis the denial of the supernatural dimension of life in The [modern] Waste Land. Reminds me of Christ's prophetic words: "Verily I say unto thee, that this night, before the cock crow, thou shalt deny me thrice." 

--- On Tue, 4/20/10, Chokh Raj <[log in to unmask]> wrote:

poetic images: our origins do not always explain our ends
  "Who is the third*who walks always beside you?"
 ["When I count, there are only you and I together
   But when I look ahead up the white road   
   There is always another one walking beside you   
   Gliding wrapt in a brown mantle, hooded   
   I do not know whether a man or a woman   
   —But who is that on the other side of you?"] 
[Incidentally, Eliot's Notes: "I am not familiar with the exact constitution of the Tarot pack of cards, from which I have obviously departed to suit my own convenience. The Hanged Man, a member of the traditional pack, fits my purpose in two ways: because he is associated in my mind with the Hanged God of Frazer, and because I associate him with the hooded figure in the passage of the disciples to Emmaus in Part V."]
*In this mystical image, the presence of the mystical number 3 in "the third" is noteworthy.