why this does not work: `use_i_i:nnn {#1}{#2}{#3}`

Why the following code does not work ?

documentclass{article}
usepackage{expl3}
ExplSyntaxOn  cs_new_protected:Npn
mycommand #1#2#3{ use_i_i:nnn {#1}{#2}{#3} }
ExplSyntaxOff
begin{document}
$mycommand{ abc } {  {def}  } { ghi }$
end{document} 

THE ERROR:

 Command Line:   D:ProgramFilesMikTexNEWmiktexbinx64xelatex.exe --interaction=errorstopmode --synctex=-1 "ideje31.tex"
Startup Folder: C:Usershynek0DesktopTH0dipl

This is XeTeX, Version 3.14159265-2.6-0.999991 (MiKTeX 2.9.7050 64-bit)
entering extended mode
(ideje31.tex
LaTeX2e <2018-12-01>
(D:ProgramFilesMikTexNEW2tex/latexbasearticle.cls
Document Class: article 2019/12/20 v1.4l Standard LaTeX document class
(D:ProgramFilesMikTexNEW2tex/latexbasesize10.clo))
(D:ProgramFilesMikTexNEW2tex/latexl3kernelexpl3.sty
(D:ProgramFilesMikTexNEW2tex/latexl3kernelexpl3-code.tex
(D:ProgramFilesMikTexNEW2tex/latexl3kernell3deprecation.def))
(D:ProgramFilesMikTexNEW2tex/latexl3backendl3backend-xdvipdfmx.def))
(ideje31.aux)
! Undefined control sequence.
mycommand #1#2#3->use_i_i:nnn 
                                {#1}{#2}{#3}
l.8 $mycommand{ abc } {  {def}  } { ghi }
                                          $
? 


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