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If we know tuples is constructed from 3 grids, we can extract the lengths using:

lF = Map[l |-> Total @ Normalize[#, Min]& @ GroupBy[#, #[[l]]& -> Most, Length]]&


ngrids = 3;

lengths = lF[tuples] @ Range[ngrids]

{6, 6, 5}

Then, we can use a combination of ArrayReshape and Extract to get the three grids:

reshaped = ArrayReshape[tuples, Append[Length @ First @ tuples] @ lengths];

positions = Map[{Splice @ RotateRight[{1, 1, All}, #], #} &] @ Range[ngrids];

Extract[reshaped, positions]

% == {grid1, grid2, grid3}

 True