The blaze broke out just after 10 p.m. on Twin Oaks Place off Kildaire Farm Road.
Five people, including one child, were able to get out of the complex safely, but a firefighter was slightly injured while battling the flames. He was taken to the hospital to be checked out and is expected to be OK.
Four apartments were damaged. Authorities said one of the units was vacant.
The cause of the fire is currently under investigation; however, it is not being called suspicious.
All three families are being helped out by the Red Cross. Only one of the families reportedly had insurance.