Taing Try, 48, was shot dead early on Sunday, according to Oum Phy, deputy police chief of Kratie province.
He is the second journalist probing Cambodia's lucrative trade in illegal timber to be killed in two years.
A former soldier, a police officer and a Phnom Penh-based military police officer -- all suspected log traders -- were arrested several hours later.
Oum Phy said the 32-year-old ex-soldier was the chief suspect in the killing, although police are still investigating the motive.
"He (the slain reporter) may have damaged the interests of the three suspects," Oum Phy added.
Sok Sovann, president of the Khmer Journalists for Democracy Association, said Taing Try was killed while he and several other reporters were investigating illegal logging -- which is widespread in the impoverished nation.
"He went to the area where he was told illegal logs were being transported," he told AFP, adding that Taing Try contributed to several local newspapers.
Sok Sovann denied local media reports that the slain journalist was trying to extort money from the suspects.
He said Taing Try had been repeatedly accused of extorting money from timber traders in the past -- although there was no evidence to back up the allegations.
Environmental activists regularly face threats in Cambodia, where land grabbing has become a major source of tension and illegal logging is rampant.