Police say the three detained in southern Maysan province Wednesday include the head of the provincial council.
A spokesman for Sadr criticized the arrests and accused the Iraqi government of targeting the cleric's supporters.
Elsewhere today, the U.S. military in Iraq said coalition forces captured two Iranian-backed Shi'ite insurgents in Baghdad.
The military says coalition troops detained the two during a raid on the home of a suspected weapons facilitator wanted in connection with attacks on coalition and Iraqi security forces.
Hoshyar Zebari, 07 May 2008In another development, Iraqi Foreign Minister Hoshyar Zebari said Jordan's King Abdullah plans to soon visit Iraq. He will be the first Arab head of state to visit the country since the U.S.-led invasion in 2003.
Zebari said Turkish Prime Minister Recep Tayyip Erdogan also is expected to visit Iraq. He said no date has been set for either trip.
Separately, the Associated Press says the White House has reported progress in 15 of 18 "benchmarks" the U.S. has set to mark political, economic and military progress in Iraq. AP, citing a May 2008 report, says the only two unsatisfactory benchmarks are enacting and implementing laws to disarm militias and distributing oil revenues.