auto arrangement of subfields in postal address fields of letters

So, if I understand correctly, if there's just two items for electronic content, we put them on the same line, but if there's three each on their own line (let's call this possibility A). The other possibility is that if there's three, the first two go together and the third goes on its own line. We'll call this possibility B.

Possibility B is the easier one. We'll can define a command, separator to be

defseparator{quaddefseparator{}}

and then write:

ifthenelse{equal{landlinephone}{}}{}%
   {landlinephsymbol~landlinephoneseparator}%
ifthenelse{equal{mobilephone}{}}{}%
   {mobilephsymbol~landlinephoneseparator}%
ifthenelse{equal{email}{}}{}%
   {emailsymbol~email}%

to get the expected result.

Possibility A is a bit trickier. We'll want to know whether we have more than two electronic contact items and then define separator appropriately. The approach for this will be to store the contact items for later expansion.

newcounter{electronicitems}

setcounter{electronicitems}{0}
ifthenelse{equal{landlinephone}{}}{defLANDLINE{}}%
   {defLANDLINE{landlinephsymbol~landlinephoneseparator}%
    stepcounter{electronicitems}}%
ifthenelse{equal{mobilephone}{}}{}%
   {defMOBILE{mobilephsymbol~landlinephoneseparator}%
    stepcounter{electronicitems}}%
ifthenelse{equal{email}{}}{defEMAIL{}}%
   {defEMAIL{emailsymbol~email}%
    stepcounter{electronicitems}}%
ifnumvalue{electronicitems}>2defseparator{}%
   elsedefseparator{quad}%
fi
LANDLINEMOBILEEMAIL

You can set up the electronic info storing to use a counter. This way you can keep track of how many items have been defined. Here's a possibly implementation:

documentclass{article}

usepackage{fontawesome}

newcommand{setlandlinephone}[1]{%
  deflandlinephone{#1}% Store landline phone number
  % Check if user supplied anything (could be empty)
  ifnumpdfstrcmp{#1}{}=0else% User did supply a non-empty landline phone number
    stepcounter{einfo}% Step einfo counter
  fi}
newcommand{setmobilephone}[1]{%
  defmobilephone{#1}% Store mobile phone number
  % Check if user supplied anything (could be empty)
  ifnumpdfstrcmp{#1}{}=0else% User did supply a non-empty mobile phone number
    stepcounter{einfo}% Step einfo counter
  fi}
newcommand{setemail}[1]{%
  defemail{#1}% Store email address
  % Check if user supplied anything (could be empty)
  ifnumpdfstrcmp{#1}{}=0else% User did supply a non-empty email address
    stepcounter{einfo}% Step einfo counter
  fi}

newcounter{einfo}
newcommand{seteinfo}{%
  ifnumvalue{einfo}=3
    % 3 elements provided; choose preference (mobile + email; ignore landline)
    faMobile~mobilephone~~faAt~email
  else% einfo < 3
    ifnumpdfstrcmp{csname landlinephoneendcsname}{}=0else
      faPhone~csname landlinephoneendcsname~~% Landline supplied
    fi
    ifnumpdfstrcmp{csname mobilephoneendcsname}{}=0else
      faMobile~csname mobilephoneendcsname~~% Mobile phone supplied
    fi
    ifnumpdfstrcmp{csname emailendcsname}{}=0else
      faAt~csname emailendcsname% Email supplied
    fi
  fi
}

begin{document}


newcommand{showeinfo}[3]{% Just for this example
  parmedskip
  setcounter{einfo}{0}% Reset electronic information counter
  setlandlinephone{#1}% Landline
  setmobilephone{#2}% Mobile
  setemail{#3}% Email
  seteinfo
}

showeinfo{123-456-7890}{}{}% Only landline
showeinfo{}{987-654-3210}{}% Only mobile
showeinfo{}{}{[email protected]}% Only email
showeinfo{123-456-7890}{987-654-3210}{}% Landline and mobile only
showeinfo{123-456-7890}{}{[email protected]}% Landline and email only
showeinfo{}{987-654-3210}{[email protected]}% Mobile and email only
showeinfo{123-456-7890}{987-654-3210}{[email protected]}% Landline, mobile and email

end{document}

In the above code, a text comparison between elements uses pdfstrcmp{<stringA>}{<stringB>}. This returns a result in the form of a number. Specifically, it returns -1 if <stringA> < <stringB> lexicographically, 0 if they're equal, or 1 if <stringA> > <stringB> lexicographically. So, with the expected outcome of this string comparison being a number, we use ifnum<string comparison>=0 to check whether the two strings are the same or not. It's specifically used to check whether the user supplied a blank entry or not, which may require expansion of the arguments - something pdfstrcmp does naturally.