ABC=[A;B;C];
d=datetime(ABC(:,1:3));
g=findgroups(d);
D=splitapply(@(x) {vertcat(x)},ABC,g);
D=cell2mat(D(cellfun(@(c)size(c,1),D)>1));
D=reshape(D.',max(histc(g,unique(g)))*size(ABC,2),[]).'
The last presumes the number of matching dates is the same in all groups to be able to reshape to a rectangular array; the previous array will be the groups with more than one element in an column-oriented array; that step without cell2mat will leave a cell array with the duplicated dates in separate cells that could have differering numbers but still with at least two elements per cell.
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