The three resort operators had ignored a court order to dismantle and remove all structures by Saturday.
Four resorts were found to have encroached on Khao Laem Ya-Mu Koh Samet National Park and their legal cases have been finalised. One has been demolished; the other three continue to operate.
National park head Arkhom Namkham said the National Parks, Wildlife and Plant Conservation Department director-general Damrong Pidech will led a team to demolish the three resorts _ Ban Ploy Samed, Unseen Samed and Mook Samed _ early this month.
The three resorts had not removed their illegal structures by the Saturday court-ordered deadline. This shows they have no intention of complying with the court order, he said.
Mr Arkhom was speaking during an inspection of the three resorts yesterday.
At the Ban Ploy Samed and Unseen Samed resorts, only a few wooden planks had been removed from the floors of some structures. At Mook Samed, only the roofs of workers' living quarters had been removed. Restaurants and other buildings were still intact.
A group of 100 resort operators and residents of Koh Samet in Rayong's Muang district have submitted a letter to national park authorities, asking to extend the demolition deadline from Sept 1 to Oct 31.
The signatories asked for permission to demolish the resorts by themselves.
Mr Arkhom said the resorts in question were asked to sign a memo to signal their agreement to an extension deadline, but they failed to sign. As a result, no agreement on the deadline extension was made, he added.
He said national park officials have been assigned to keep a close watch on the three resorts and to ask them to urgently remove their structures and belongings from the encroached areas.
The national park had earlier put up demolition notices in front of the three resorts, but the notices had disappeared, Mr Arkhom said.
The national park would also file charges against 57 operators of buildings, mostly shops and restaurants, on Mae Ramphueng beach in Rayong They are considered to be encroaching on the national park, Mr Arkhom said.
Last month, Rayong province, local administration organisations and operators agreed to transfer the cases of the 57 building structures to the National Parks Department.
The department has denied receiving notice of this and said it was working on setting out regulations for managing the area.