Código HTML:
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<html> <head> <script language="javascript"> function suma(form){ form.total.value = form.a.value + form.b.value + form.c.value; return; } </script> </head> <body> <form> <input type="text" name="a" onkeyup=suma(this.form)> <input type="text" name="b" onkeyup=suma(this.form)> <input type="text" name="c" onkeyup=suma(this.form)> <input type="text" name="total"> </form> </body> </html>