A 34-year-old collapsed in Blackburn on 24 January and later died in hospital. Four days later, a 36-year-old was found dead in Colne.
A third man, aged 56, died in Accrington on Friday. The deaths are all being treated as unexplained.
Lancashire Police said it was "unclear if there was a direct link" but warned drug users to be "cautious".
An investigation into whether there was a contaminated batch of heroin in the area would be carried out, a police spokesperson added.
A further two men, from Blackburn and Accrington, were admitted to hospital with similar symptoms.
Det Insp Tim McDermott said "It remains unclear if there is any direct links between the sad deaths of these three men.
"However, given they have taken place in a comparatively short period of time and all within East Lancashire I think it's right that we remind drug users within our communities the need to be particularly cautious."
Anyone with information is asked to contact police on 101 or Crimestoppers.
Three men have died of suspected overdoses in East Lancashire after taking heroin, police have said.