A Chinese ship reported picking up electronic signals twice, once on Friday and again on Saturday in the southern Indian Ocean.
On Sunday, an Australian ship carrying sophisticated deep-sea sound equipment picked up a third signal in a different part of the massive search area.
"This is an important and encouraging lead, but one which I urge you to treat carefully," retired Australian Air Chief Marshall Angus Houston, who is coordinating the search, told reporters in Perth.
Houston stressed that the signals had not been verified as being linked to Flight 370, which was traveling from Kuala Lumpur, Malaysia, to Beijing when it disappeared March 8 with 239 people on board. Experts, meanwhile, expressed doubt that the equipment aboard the Chinese ship was capable of picking up signals from the black boxes.
"We have an acoustic event. The job now is to determine the significance of that event. It does not confirm or deny the presence of the aircraft locator on the bottom of the ocean," Houston said, referring to each of the three transmissions.
"We are dealing with very deep water, we are dealing with an environment where sometimes you can get false indications," he said. "There are lots of noises in the ocean, and sometimes the acoustic equipment can rebound, echo if you like."