Yo lo haría asi:
Código PHP:
<html>
<head>
<script language="JavaScript">
function imprimir(){
var vnt1 = open("uno.htm", "uno"); vnt1.print(); vnt1.close();
var vnt2 = open("dos.htm", "dos"); vnt2.print(); vnt2.close();
var vnt3 = open("tre.htm", "tre"); vnt3.print(); vnt3.close();
}
</script>
</head>
<body>
<form name="frm">
<input type="button" name="imp" value="Imprimir" onclick="imprimir()">
</form>
</body>
</html>
Saludos.