documentclass{article}
newlength{myLength}
begin{document}
setlength{myLength}{3pt plus2pt minus1pt}
setlength{myLength}{-myLength}
themyLength
end{document}

-3.0pt plus -2.0pt minus -1.0pt

-3pt plus -2pt minus -1pt

<natural width> stretchable <stretch component> shrinkable <shrink component>