The Malta-flagged Kalamos was anchored and awaiting fresh cargo from Qua Iboe, an oil terminal in Nigeria's southeastern region operated by ExxonMobil, when it was boarded yesterday.
Two of the three hostages are Greek, and Greeks make up 10 out of the total 23 crew, a spokeswoman for the coastguard told AFP.
The hijacking of the 15-year-old tanker took place in the Gulf of Guinea, which experts have said is the new epicentre of piracy in Africa.
It said the pirates operating off the coast of Nigeria, Togo and Benin are heavily armed and violent, and often hold crews hostage for several days.