Recently we update Windows 2003 to Windows 2012 R2 with a TrustRelationship with a samba 3, to keep it working you have to modify the windows registry on the windows 2012 machine,
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanManWorkstation\Parameters]
“DNSNameResolutionRequired”=dword:00000000
“DomainCompatibilityMode”=dword:00000001
And having done that, it’s the same process to join windows 7 or superior to a samba 3 domain.
This is a excelent reference.