Código HTML:
<html> <head> <script language="JavaScript"> function calcularSuma(){ var valor1 = 0; var valor2 = 0; var valor3 = 0; if(document.formulario.valor1.value != "") valor1 = Number(document.formulario.valor1.value); if(document.formulario.valor2.value != "") valor2 = Number(document.formulario.valor2.value); if(document.formulario.valor3.value != "") valor3 = Number(document.formulario.valor3.value); document.formulario.resultado.value = valor1 + valor2+ valor3; } </script> </head> <body> <form name="formulario"> <table> <tr><td><input type="text" name="valor1" onBlur="calcularSuma()"/></td></tr> <tr><td><input type="text" name="valor2" onBlur="calcularSuma()"/></td></tr> <tr><td><input type="text" name="valor3" onBlur="calcularSuma()"/></td></tr> <tr><td><input type="text" name="resultado"></td></tr> </table> </form> </body> </html>