hmm, if you really only need this for three columns, I suggest you use the three nodes of str manip, for loop on column list is unfortunately not very easy to handle. If there are many more columns, then I'll describe you a possible flow tomorrow.
I think it would be the easiest if we had minLength/maxLength for String columns as an aggregation function (so it could be used in GroupBy for the use case you described). I have checked and it seems it is not part of the base KNIME installation, maybe someone already implemented it though.