What are the three locations? And, What is the best way to obtain them?

Unfortunately, the only places the Morag Tong helms exist is on the heads of Morag Tong assassins. Specifically, Vaveli Dralas in Sadrith Mora, Namanian Facian also in Sadrith Mora and Taros Dral in Vivec. Unless you're willing to kill one of them, the only option is to cheat or mod.

Yeah I Morrowind Wiki'd it, after you do the quest with the treated bittergreens, and recieve the ring of kahjiit, that member with the helmet becomes useless, so i slew him and took the helmet.