In[]:=
DM={{{0,0},{0,0}},{{0,0},{0,0}}}
In[]:=
DM
Out[]=
{{{0,0},{0,0}},{{0,0},{0,0}}}
In[]:=
For[i1=1,i1<3,i1++,For[i2=1,i2<3,i2++,For[xx=1,xx<3,xx++,\If[xx==1,DM[[i1]][[i2]][[xx]]=A,DM[[i1]][[i2]][[xx]]=B]]]]
In[]:=
DM
Out[]=
{{{B,B},{B,B}},{{B,B},{B,B}}}
In[]:=
For[i1=1,i1<3,i1++,For[i2=1,i2<3,i2++,For[xx=1,xx<3,xx++,\If[xx==1,DM[[i1]][[i2]][[1]]=A,DM[[i1]][[i2]][[2]]=B]]]]
In[]:=
DM
Out[]=
{{{A,B},{A,B}},{{A,B},{A,B}}}
In[]:=
For[p=1,p<3,p++,For[q=1,q<3,q++,For[xx=1,xx<3,xx++,\If[xx==1,DM[[p,q,xx]]=A,DM[[p,q,xx]]=B]]]]
In[]:=
DM
Out[]=
{{{B,B},{B,B}},{{B,B},{B,B}}}
In[]:=
DM={{{0,0},{0,0}},{{0,0},{0,0}}};
In[]:=
Do[DM[[i1,i2,i3]]=If[i3==1,A,B],{i1,1,2},{i2,1,2},{i3,1,2}]
In[]:=
DM
Out[]=
{{{A,B},{A,B}},{{A,B},{A,B}}}