DataA={{"A"}};
GridA = DataGrid[{2},{5},{0,0},{{1,1},{1,1}},<|"border"->"*","alignment"->{1,-1}|>,{DataA}];
DataB = {{"B"},{"B"}};
GridB =
  DataGrid[{4,3},{4,8},{2,3},{{0,0},{0,0}},<|"background"->".","alignment"->{0,1},"dividers"->{"-","|","+"}|>,{DataB,DataB,DataB}];
GridC =
  DataGrid[{2,3},{2,3},{{0,1},{2,3}},<|"border"->"@","alignment"->{0,0},"dividers"->{"/","/","/"}|>,{GridA,GridB,GridA,GridB,GridA}];

DataGrid[{2,3},{2,12},{{0,0},{0,0}},<||>,{{{"A"}},{{"B"}},{{"C"}},GridA,GridB,GridC}]//AsString