This tutorial is based on jquery ,you can type text in more than one text box at a time without copy and pasting the content.

$('#one').keyup(function(){
  $('#two').text( $(this).val());
  $('#three').text( $(this).val());
});

what this above jquery code will do is , it will be embedded with the html code and there are 3 ID's for 3 text area's #one , #two and #three . the function keyup() will trigger on key up of the ID #one (i.e) the first text area , the same text will be replicated in other two text area's using

$('#two').text( $(this).val());

See the Pen ogovKM by Arun Yokesh (@yokesh) on CodePen.

if you are using the input box means you have to use the following function called val()

$('#input_box_id').val( $(this).val());

$(this) always points to the parent function in which the $(this) is used.