Did you try running the Homegroup Troubleshooter (run as Administrator) from each computer. If the Troubleshooter can't fix it, it usually suggests leaving the Homegroup on each of the computers and starting over creating the Homegroup. Sometimes it takes a restart after running the Troubleshooter for the new settings to take affect. Perhaps re-create the Homegroup using one of the hard wired desktops.
Also, are all 3 computers using the final version of W7? There were several Homegroup issues in the beta and RC releases.