Mrthdrb

I want to draw an incomplete cube, as in the following figure.

But, I can only draw a complete cube, see below.

documentclass[12pt]article
usepackagetikz
usepackageverbatim
usetikzlibrarypositioning
begindocument
begintikzpicture[on grid][scale=1.4]
 shade[yslant=-0.5,right color=white, left color=white]
 (0,0) rectangle +(3,3);
 draw[yslant=-0.5] (0,0) grid (3,3);
 shade[yslant=0.5,right color=white,left color=white]
 (3,-3) rectangle +(3,3);
 draw[yslant=0.5] (3,-3) grid (6,0);
 shade[yslant=0.5,xslant=-1,bottom color=white,
 top color=white] (6,3) rectangle +(-3,-3);
 draw[yslant=0.5,xslant=-1] (3,0) grid (6,3);
endtikzpicture
enddocument

Just for fun: everything is in 2D

documentclass[tikz]standalone
begindocument
begintikzpicture
draw (0,0) rectangle (1,1) (1,1) rectangle (2,2) (0,1) rectangle (1,2) (0,2) rectangle (1,3) (1,0) rectangle (2,1) (2,0) rectangle (3,1);
draw (3.6,1.6)--(3.6,.6)--(3,0)--(3,1)--cycle--(3.6,2.6)--(2.6,2.6)--(2.6,3.6)--(.6,3.6)--(0,3);
draw (2,1)--(2.4,1.4) (2,2)--(2.6,2.6) (1,2)--(1.4,2.4) (1,3)--(1.6,3.6) (2.4,3.4)--(2.6,3.6) (3.4,2.4)--(3.6,2.6); 
draw (2.4,1.4) rectangle (3.4,2.4) (1.4,2.4) rectangle (2.4,3.4);
draw (.4,3.4)--(1.4,3.4) (3.4,1.4)--(3.4,.4) (.2,3.2)--(1.2,3.2)--(1.2,2.2)--(2.2,2.2)--(2.2,1.2)--(3.2,1.2)--(3.2,.2);
endtikzpicture
enddocument

With colors

documentclass[tikz]standalone
begindocument
begintikzpicture
fill[black!70] (3,0)--(3.6,.6)--(3.6,2.6)--(3.4,2.4)--(3.4,1.4)--(3,1)--cycle (2.4,2.4)--(2.6,2.6)--(2.6,3.6)--(2.4,3.4)--(2.4,1.4)--(2,1)--(2,2)--cycle (1,2)--(1.4,2.4)--(1.4,3.4)--(1,3)--cycle;
fill[black!30] (0,3)--(.6,3.6)-_(2.6,3.6)--(2.4,3.4)--(1.4,3.4)--(1,3)--cycle (2.4,2.4)--(2.6,2.6)--(3.6,2.6)--(3.4,2.4)--(1.4,2.4)--(1,2)--(2,2)--cycle (2,1)--(2.4,1.4)--(3.4,1.4)--(3,1)--cycle;
draw (0,0) rectangle (1,1) (1,1) rectangle (2,2) (0,1) rectangle (1,2) (0,2) rectangle (1,3) (1,0) rectangle (2,1) (2,0) rectangle (3,1);
draw (3.6,1.6)--(3.6,.6)--(3,0)--(3,1)--cycle--(3.6,2.6)--(2.6,2.6)--(2.6,3.6)--(.6,3.6)--(0,3);
draw (2,1)--(2.4,1.4) (2,2)--(2.6,2.6) (1,2)--(1.4,2.4) (1,3)--(1.6,3.6) (2.4,3.4)--(2.6,3.6) (3.4,2.4)--(3.6,2.6); 
draw (2.4,1.4) rectangle (3.4,2.4) (1.4,2.4) rectangle (2.4,3.4);
draw (.4,3.4)--(1.4,3.4) (3.4,1.4)--(3.4,.4) (.2,3.2)--(1.2,3.2)--(1.2,2.2)--(2.2,2.2)--(2.2,1.2)--(3.2,1.2)--(3.2,.2);
endtikzpicture
enddocument

You can define a single cube as pic and place it where you want (starting from bottom back to top front).

documentclass[tikz,border=7pt]standalone
tikzset
 cube/.pic=
 draw[fill=black!20] (0,1,0) -- (0,1,1) -- (1,1,1) -- (1,1,0);
 draw[fill=black!50] (1,0,0) -- (1,0,1) -- (1,1,1) -- (1,1,0);
 draw[fill=white] (0,0,0) rectangle (1,1,0);
 

begindocument
 tikz[z=(.4,.3)]pathpgfextradef~piccube % <--- to make the code shorter
 (2,0,2)~(2,0,1)~(0,0,0)~(1,0,0)~(2,0,0)~
 (2,1,2)~(1,1,1)~(0,1,0)~(1,1,0)~
 (0,2,2)~(1,2,2)~(0,2,1)~(0,2,0)~;
enddocument

I would like to argue that one should use orthographic projections and write it in such a way that one can change the view angle. There are already many posts on this, perhaps most impressingly this one. The idea to use pics for the unit cubes to draw 3d cubes with some little cubes missing is also not new, it has been used here, where the cubes are rotatable in 3d. I just recycled the code to get

documentclass[tikz,border=3.14mm]standalone
usepackagetikz-3dplot
usetikzlibrarycalc
tikzsetplane/.style n args=3insert path=%
#1 -- ++ #2 -- ++ #3 -- ++ ($-1*#2$) -- cycle,
unit xy plane/.style=plane=#1(1,0,0)(0,1,0),
unit xz plane/.style=plane=#1(1,0,0)(0,0,1),
unit yz plane/.style=plane=#1(0,1,0)(0,0,1),
get projections/.style=insert path=%
let p1=(1,0,0),p2=(0,1,0) in 
[/utils/exec=pgfmathtruncatemacroxprojsign(x1)xdefxprojxproj
pgfmathtruncatemacroyprojsign(x2)xdefyprojyproj
pgfmathtruncatemacrozprojsign(cos(tdplotmaintheta))xdefzprojzproj],
pics/unit cube/.style=code=
path[get projections];
draw (0,0,0) -- (1,1,1);
ifnumzproj=-1
 path[3d cube/every face,3d cube/xy face,unit xy plane=(0,0,0)]; 
fi
ifnumyproj=1
 path[3d cube/every face,3d cube/yz face,unit yz plane=(1,0,0)]; 
else
 path[3d cube/every face,3d cube/yz face,unit yz plane=(0,0,0)]; 
fi
ifnumxproj=1
 path[3d cube/every face,3d cube/xz face,unit xz plane=(0,0,0)]; 
else
 path[3d cube/every face,3d cube/xz face,unit xz plane=(0,1,0)]; 
fi
ifnumzproj>-1
 path[3d cube/every face,3d cube/xy face,unit xy plane=(0,0,1)]; 
fi
,
3d cube/.cd,
xy face/.style=fill=gray!20,
xz face/.style=fill=gray!50,
yz face/.style=fill=gray!90,
every face/.style=draw,very thick

begindocument
foreach Angle in 5,15,...,355 
tdplotsetmaincoords60Angle % the first argument cannot be larger than 90
begintikzpicture[line join=round]
 pgfmathtruncatemacroNumCubes7
 path[use as bounding box] (-NumCubes/2-3,-NumCubes/2-2) 
 rectangle (NumCubes/2+3,NumCubes/2+4);
 beginscope[tdplot_main_coords]
 pgfmathtruncatemacroNextToLastNumCubes-1
 path[get projections];
 ifnumyproj=1
 defLstX1,2,3
 else 
 defLstX3,2,1
 fi
 ifnumxproj=-1
 defLstY1,2,3
 else 
 defLstY3,2,1
 fi
 foreach X in LstX
 foreach Y in LstY
 ifnumY=3
 pgfmathtruncatemacroZmax5-max(X,2)
 else
 pgfmathtruncatemacroZmax4-X
 fi
 foreach Z in 1,...,Zmax
 path (X-2,Y-2,Z-1) picunit cube;
 
 endscope
endtikzpicture
enddocument