Here is the file that I have been working with:

\installlanguage[packed][en]
\setuplanguage[packed][spacing=packed]
\startsetups[tightspace]
  \spaceskip 0.5\interwordspace plus .5\interwordstretch minus
\interwordshrink
\stopsetups

\def\dostartbibitem[#1]{\startBibItem[title={#1},reference={#1}]}
\def\startbibitem{\dosingleempty\dostartbibitem}
\def\stopbibitem{\stopBibItem}

\definedescription[BibItem][
width=broad,
margin=1.5pc,
indenting={no},
indentnext=no,
alternative=hanging,
hang=1,
headcommand=\gobbleoneargument,
align=right,
before={\directsetup{tightspace}\bgroup\language[packed]},
after={\egroup},
]

\definereferenceformat [cite]  [type=title,left={},right={}]
\definereferenceformat [bibpage]   [type=page]

\starttext

\startbibitem[A, B, and C]\stopbibitem % TEST by commenting these
%\startbibitem[A{,} B{,} and C]\stopbibitem % lines out

\startbibitem[ABC]\stopbibitem
\startbibitem[A]\stopbibitem
\startbibitem[B]\stopbibitem
\startbibitem[C]\stopbibitem

* \cite[A, B, and C] \qquad{\red output should  be: A{,} B{,} and C}

**\cite[ABC] \qquad{\red output should be: ABC}

***\cite[A], \cite[B], \cite[C]  \qquad{\red output should be: A, B, C}

****\cite[A]\cite[B]\cite[C] \qquad{\red output should be: ABC}

\stoptext


On Wed, Feb 22, 2017 at 11:44 AM, Hans Hagen <pragma@wxs.nl> wrote:

> On 2/22/2017 4:50 PM, Alan Bowen wrote:
>
>> Hi, Hans—
>>
>> The problem is the line \startbibitem[A, B, and C]\stopbibitem. I can
>> either keep it and any others like  \cite[A], \cite[B] or
>> \cite[A]\cite[B] are messed up or I can comment it out and these same
>> citations work.
>> \startbibitem[A{,} B{,}and C]\stopbibitem (as you suggested) does allow
>> the other citations to work. But, then, neither \cite[A, B, and C] nor
>> the \cite[A{,} B{,} and C] works.
>>
>
> so we need a small example
>
>
> -----------------------------------------------------------------
>                                           Hans Hagen | PRAGMA ADE
>               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>        tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
> -----------------------------------------------------------------
> ____________________________________________________________
> _______________________
> If your question is of interest to others as well, please add an entry to
> the Wiki!
>
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/list
> info/ntg-context
> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
> archive  : https://bitbucket.org/phg/context-mirror/commits/
> wiki     : http://contextgarden.net
> ____________________________________________________________
> _______________________
>