When you enter something that doesn't match, it often defaults to NaN for numeric columns,the simplest way to fix it is it set ColumnFormat to 'char' or 'string' for the column where you want to type [1,2,3]. MATLAB will then treat it as text, not try to convert it to a single number, and thus won't produce NaN. You'll then process the text yourself in your code.
Hope it helps.
