Two pine trees were attacked last month in West Cliff Green, Bournemouth, Dorset, close to the site of the former St George's Hotel - both trunks had up to 15 holes drilled in them.
A third pine has since been attacked and a "crystal-like" substance has been found on its trunk.
The substance is now being tested.
Andy McDonald, head of parks at Bournemouth Borough Council, said he had "strong suspicions" the trees had been poisoned - possibly by pouring a substance into the drilled holes.
The authority said it also hoped to install CCTV cameras in the area in a bid to prevent further tree attacks.
It added the latest attack was likely to have been carried out in the past two weeks.
Dorset Police said it was investigating the damage, but no arrests had been made.
Chris Colledge, from West Cliff Residents' Association, said: "The local community are absolutely lost for words as to how anybody could want to destroy such a beautiful pine."
The two affected maritime pines and one Scots pine, which are on public land, are each up to 40 years old but are not subject to preservation orders.
Tests are being carried after three trees are believed to have been poisoned in a seaside resort, a council has said.