Const sPattern As String = "\d{1,3}((\.)|(\ \.)|(\ [A-Z,a-z]))"

You have some optional arguments (I think, I don't recognize the syntax)
that I've removed.  This just uses the pipe to separate the three choices.

Your examples do not match your description in several aspects:

Your description says you are looking for **three** digits;

Assuming the "\" should be before the "d", your examples will match 1, 2 or 3
digits.

In your first example, you are using a positive lookahead assertion which means
you will only match the digits, and not the subsequent period.

In your second example, you don't use the positive lookahead assertion, so the
subsequent <space dot> will be matched also.  This seems inconsistent.  And
also, enclosing the <space> and the <.> within a character class is
unnecessary.

Finally your third pattern, in the area following the digits, matches a
<space>, but the subsequent series within the character class do not make up a
legitimate pattern, because the backslash cannot be followed by an "A" when
within a character class.  Also, note that you have a <comma> within the
character class, so you would be also matching a <comma>, which is not
something you stated in your description

Going by your description, if you want to match a series of three digits,
followed by one of those three sequences you describe, then:

"\d{3}(\.|\s\.|\s[A-Za-z])"

or, if you want to match ONLY the digits, provided they are followed by one of
those other three sequences, then:

"\d{3}(?=\.|\s\.|\s[A-Za-z])"

--ron