The incidents brought to six the number of ships menaced in the Gulf of Aden in the past month, said Noel Choong, head of the IMB reporting centre.
A German-operated cargo ship, flying the flag of Antigua and Barbuda, was the last to be targeted, at 9:45 am GMT, while the Iranian and Japanese tankers were hijacked earlier in the day between 2:00 am GMT and 3:00 am GMT.
Choong said the attacks took place very near to each other, but he was not
able to say whether the same group was responsible.
"Whether it's a different group of pirates, we can't tell until an investigation is carried out. We have sent out an urgent warning to all ships travelling through the Gulf of Aden," he told AFP. "We want to pressure the United Nations and the international community to do something about it, to take steps to stop this menace."
But he said there was little that could be done without UN intervention since Somalia has no central government.
The waters off Somalia are among the most pirate-infested in the world, with the IMB reporting 24 attacks in Somalia between April and June this year.
The German Foreign ministry said on Thursday it was investigating the latest report.