Span does not have 'change' event by default. But you can add this event manually.
Listen to the change event of span.
$("#span1").on('change',function(){
//Do calculation and change value of other span2,span3 here
$("#span2").text('calculated value');
});
And wherever you change the text in span1. Trigger the change event manually.
$("#span1").text('test').trigger('change');