provato così?

codice:
<form name="form1">
<input type="text" onKeyPress="if (event.keyCode==46) this.form.campo2.focus();" name="campo1" maxlength="3" size="3">
<input type="text" onKeyPress="if (event.keyCode==46) this.form.campo3.focus();" name="campo2" onFocus="if (this.value=='.') this.value='';" maxlength="3" size="3">
<input type="text" onKeyPress="if (event.keyCode==46) this.form.campo4.focus();" name="campo3" onFocus="if (this.value=='.') this.value='';" maxlength="3" size="3">
<input type="text" name="campo4" maxlength="3" size="3" onFocus="if (this.value=='.') this.value='';" maxlength="3" size="3">
</form>
ciao