From Diablo Wiki
01/12/2010: "Sockets: As of v1.09 in D2X you can find helms with 3 sockets only on Hell. Up to 2 sockets can be found in Normal and Nightmare. " Socketing rules have clearly been relaxed as of 1.13, as it is possible to get 3-socket helms from NM/Act III onwards, as well as 4-socket armour. This is both on regular and class-specific helms.