Two years ago, the Boeung Kak lake area in central Phnom Penh was home to 4,000 families.
Land disputes continue to be an ongoing problem for many Cambodians, who settled on land after the Khmer Rouge era.
The union was planning to demonstrate against rising prices of food, fuel and other living expenses.
The 58-article law was sent to the Council of Ministers for approval last week, after which it will pass to the National Assembly.
The July meeting in Chicago was to set a strategic plan for the organization for the next two to three years.
The government says it needs the NGO law to better manage the thousands of organizations operating in the country.
The three men, who deny assaulting the police, say they are afraid the summonses will mean their arrest.
Only three-quarters of those surveyed in 2010 had heard of parliament, but of those, only two-thirds knew what it does.
About 250 families are laying claim to some 800 hectares of land they say was taken by a private company in 2005.
The US-based rights group said the charges against Leang Sokchouen had changed during appeal to incitement under the new penal code.
Leang Sokchouen was given a two-year prison sentence and $500 fine in August 2010.
Union leaders say the draft of the law will tighten restrictions on organized labor and create harsh punishments for unionists.