Conservation director Martin Harper said a male disappeared from one nest three weeks ago with males at two others not being seen for a week.
Male hen harriers disappearing while part of an active nesting attempt is "exceptionally unusual" he said.
In the absence of males to provide food, the hungry females were forced to abandon their nests or starve.
The RSPB lists the hen harrier as a red status species, meaning it is threatened and populations have suffered severe declines in numbers.
Last year there were only two breeding pairs in the Forest of Bowland.
The area, owned by United Utilities, has been called "the last stronghold in England" for the birds by conservationists.
The RSPB has worked with the water company to protect the birds there for more than 30 years.
Mr Harper said it was "very early days" to know what happened to the birds, but he urged anyone who might have seen anything suspicious to contact police.
Source: BBC Nature
Three rare birds of prey have disappeared from the Forest of Bowland in Lancashire, the RSPB has said.