Li3 is an abbreviation for the trilogarithm function, i.e. Li3 = PolyLog[3, #]&.
Li3
Li3 = PolyLog[3, #]&
Overview, Li2.
Li3[x]
\text{Li}_3(x)
Li3 // StandardForm (*PolyLog[3, #1] &*)
D[Li3[x], x]
\frac{\text{Li}_2(x)}{x}
Integrate[Li3[x]/x, x]
\text{Li}_4(x)