documentclass{article}

usepackage[showframe,papersize={5.5in,8.5in}]{geometry}
newlength{testwd}

newcommand{entry}[4][]{%
    setbox1=hbox{strutbfseries#2}%
    setbox2=hbox{strutitshape#3}%
    setbox3=hbox{strut#4}%
    setlength{testwd}{ifdimwd1>wd3 wd1elsewd3fi}% Which is longer, L or R? Use it.
    noindent%
        if###1##elsellap{#1thinspace}fi
        hbox to 0pt{box1hss}%
        hfill
        %% If the longer side (L or R) is longer than the half the remaining area around
        %% the center text, drop the center text.
        ifdimdimexpr testwd - (linewidth - wd2)/2relax>0ptlowerbaselineskipfi
        hbox{box2}%
        hfill
        hbox to 0pt{hssbox3}%
    par
}

begin{document}

entry{Left side text}{Center Text}{Right text}

entry{Left side text that is rather long}{Center Text}{Right side text}

entry{Left side text}{Center Text}{Right side text that is rather long}

end{document}

documentclass{article}

usepackage[showframe,papersize={5.5in,8.5in}]{geometry}

usepackage{makecell}

newlength{LeftItem}
newlength{CentreItem}
newlength{RightItem}

newcommand{entry}[3]{%
  par
  settowidth{LeftItem}{bfseries #1}%
  settowidth{CentreItem}{itshape #2}%
  settowidth{RightItem}{#3}%
  noindent
  makebox[0pt][l]{bfseries #1}hfill
  makebox[0pt]{%
    ifdimLeftItem>.5dimexprlinewidth-CentreItemrelax
      makecell[t]{strut \ itshape #2}%
    else
      ifdimRightItem>.5dimexprlinewidth-CentreItemrelax
        makecell[t]{strut \ itshape #2}%
      else
        itshape #2%
      fi
    fi
  }hfill
  makebox[0pt][r]{#3}%
  par
}

begin{document}

entry{Left side text}{Center Text}{Right text}

entry{Left side text that is rather long}{Center Text}{Right side text}

entry{Left side text}{Center Text}{Right side text that is rather long}

end{document}

newcommand{entry}[3]{%
  % ...
  settowidth{CentreItem}{itshape ~#2~}% Add space on either side of centre item
  % ...
}