It looks like you just need to put the following i the callback of your editbox (I'm assuming you meant editbox instead of textbox):
function myEdit_Callback(hObject,eventdata,handles)
nRows = str2num(get(hObject,'String'));
dat = cell(nRows,3);
set(handles.myTable,'Data',dat)
guidata(hObject,handles)
That, of course, will overwrite any data entered in your table with a blank table. If you want to keep your data, it'll be a bit more complicated:
function myEdit_Callback(hObject,eventdata,handles)
oldDat = get(handles.myTable,'Data');
nRows = str2num(get(hObject,'String'));
if nRows > size(oldDat,1)
dat = cell(nRows,3);
dat(1:nRows,:) = oldDat;
elseif nRows < size(oldDat,1)
dat = oldDat(1:nRows,:);
end
set(handles.myTable,'Data',dat)
guidata(hObject,handles)
