An arborist's report to the Dunedin City Council said a further two might not survive another year and may be removed.
The trees suffered root damage from the construction of retaining walls and a fungal disease that swept through the stand a few years ago.
Council parks manager Lisa Wheeler said an assessment by contractors identified the three trees as a risk to the public and they recommended they be removed as soon as practicable.
Two of the trees are in the lower quadrant and one, transplanted about three years ago, is at the George St end of the upper level.
Mrs Wheeler said they would be removed in sections, in a process timed to avoid events being held in the Octagon and cruise ship visits .
The two trees identified as in decline, opposite the Dunedin Public Art Gallery, would continue to be part of a health treatment programme which had helped most of the stand recover from root damage and the virus.
The dying trees would be monitored and their health reassessed in winter and again in spring, but the arborist's report said it was likely they, too, would die, Mrs Wheeler said.
''By next winter, they would have had a breathing space after a dormant period, they would have had treatment through the year and it could give them a boost.''
The dead trees would be cut down to ground level and the wood made available for artisans.
The removed trees would not be replaced until an urban design review of the Octagon had been completed.
The outcome of that review would enable the council to decide if the trees would be replaced and, if so, what species would be planted, Mrs Wheeler said.
