"The three Germans were released on Friday at 10 a.m. after mediation by a tribal dignitary from the Bani Dhabyan region," where the abductors are from, the source said on condition of anonymity.
The three captives, a German woman working in Yemen and her visiting parents, were seized on Sunday in the Bani Dhabyan region some 80 kilometres (50 miles) east of the capital Sanaa.
A tribal official said initially that one of the kidnappers was demanding $200,000 to recompense him for lost land as well as the release of his brother and son who were arrested four months ago over a land dispute.
On Thursday, a tribal source told AFP the tribesmen holding the family hostage were also demanding the release of two Yemenis detained in the United States for supporting Al-Qaeda.
Unconfirmed reports on the Yemeni Internet site Marebnews said Tam was being paid a ransom of $100,000 to free the captives and reassurances that he would not face prosecution.
Tribes have abducted more than 200 foreigners over the past 15 years in a bid to extract concessions from the central government whose writ extends with difficulty over the lawless countryside.
All foreign hostages have been freed unharmed except for three Britons and an Australian seized by Islamist militants in December 1998. They were killed when security forces stormed the kidnappers' hideout.