Fire officials say the blaze broke out at 9:45 a.m. Thursday at the Wood River Village Retirement Community in Bensalem.
Four apartments at the facility at Hulmeville Road and Bensalem Road were evacuated as crews worked to bring the fire under control.
Investigators say the fire appears to have started in a heating unit in apartment F-111 and spread into the walls.
Three employees and a 79-year-old resident were taken to area hospitals to be treated for smoke inhalation and burns.
Eight residents have been moved to other apartments within the complex.