Windows

You must create the user account in each domain. Then the user can simply type in thier user name and password and domain name and join that domain.

You either have to be using DHCP in all 3 domains, or you must statically assign an IP address to the laptop. If statically assigning an IP address, all 3 domains must be in the same subnet so the IP address will work in all 3 places. In Network Neighborhood, make sure to set your Primary Network Logon to "Client for Microsoft Networks" (done on the laptop). That way, the user will be prompted for a username, password, and domain during bootup. As long as the network protocols are the same in all 3 domains, then everything should work. Good luck.

Have you try creating a 3 profiles, one for each domain, by using slightly different logon names for example johnG for John in Gerogia, JohnN for New York, etc...
With separate profiles on the machine, using the appropriate profile he will join each domain.
Now, when he goes to each of the sites, he uses the appropriate profile and log into the appropriate domain.

Good Luck!

Create three seperate user account of the same name and password in each domain.

Then it should help u (Password authentication)

Create 3 accounts with the same username and password in eack domain then have the user type in the domain when loggin on. You will also need all networks to be DHCP or set all networks to reserve his IP for only his PC.