The wharf bombing alone left 15 people dead and 40 others wounded.
Commemorating the third anniversary of the twin attacks last Sunday, City Mayor Rodrigo Duterte went to the Sasa wharf, lit candles and offered prayers and flowers.
Duterte said the government has been doing everything to catch the perpetrators.
"We know who the plotters were, but we are having a difficult time collecting and pooling hard evidence that would convict (them) once and for all," he said.
Authorities have charged five suspects in connection with the bombing.
The suspects have repeatedly denied the charges, claiming they are merely fall guys who were tortured to own up to the attack.
Duterte, however, assured Davaoeños that he is doing everything to protect them, in the hope of preventing a repeat of the tragedy.
"We are doing everything possible so there would be no repeat of this kind of carnage," Duterte said during his weekly television program.