Fathers Pang Yongxing, 30, Ma Shunbao, 50, and Wang Limao, 32, all underground Roman Catholic clergy members in Baoding city in the northern province of Hebei, were sentenced on July 7 for "disturbing the peace of society", the Connecticut-based Cardinal Kung Foundation said.
Police arrested Wang as he delivered mass on Palm Sunday in March, Ma while offering Easter Sunday mass a week later, and Pang in his home on December, 2001, it said in a faxed statement received on Saturday.
It said the three were being held at the Balizhuang labour camp in Baoding, around 150 km south of Beijing.
Phone calls to the camp were not answered.
"To sentence these Roman Catholics under the 'cult' law and to punish them for this in labour camps is a transparent example that there is no religious freedom in China," said the foundation's president Joseph Kung.
"It makes a mockery of the 'religious freedom' clause in the Chinese constitution."
Rights groups say China has used cult laws, enshrined as part the government's fight against banned spiritual movement Falun Gong, to ratchet up its crackdown on Christians and Muslims outside the State-approved religious bureaucracy.
The Holy See estimates there are eight million Chinese Catholics loyal to the Pope, compared to only five million in the state-backed Catholic Church.
Six underground Catholics from the coastal province of Zhejiang were arrested for "illegal pilgrimages" to Baoding but released after they paid a total of 30,800 yuan ($3,850) in fines, the group added.
Another dozen were unable to pay the fines and still in custody, it said.
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