Thank you 'tuxembb'.
I was connecting the devices this way (croosover cable). I don´t know why but, the older digi was received with fixed an IP and the two new with DHCP.
I installed 'Tftpd' and seted up a DHCP Server as in http://www.digi.com/support/kbase/kbaseresultdetl?id=843
and the Digi got an IP, and then I was able to find it using "Device Discovery Utility for Windows".
Thank you 'tuxembb', best regards.