When Boston police Officer Luis Lopez pulled up the house at 8 Montello St. in Dorchester Saturday, he could see that the three-alarm fire had grown intense.

“You could see the flames punching through the roof and it didn’t look good,’’ Lopez said in a statement released by the Police Department. “As soon as I exited my cruiser, people started pointing to the building and yelling that there were still people inside.’’

Lopez said he immediately entered the building and started knocking on doors, making sure people got out of the building.

The Dorchester fire, which happened about 8 p.m., was one of three blazes to break out in a span of six hours stretching into Sunday morning. In total, at least 49 people were displaced — including 14 adults and four children at Montello Street.

One firefighter suffered a back injury, and was taken to a hospital by emergency medical personnel, according to fire officials. The damage was estimated at $500,000.

Montello said he didn’t hesitate to enter the building despite the flames.

“This is what we signed up for and I’m grateful I was able to make a difference today,’’ said Lopez.

Also Saturday, firefighters responded at 7 p.m. to a report of a building fire at 12 and 16 Woodcliff St. in Roxbury. The fire displaced 15 adults and seven children. Damages were estimated at $200,000.

Then, fire crews also responded to a three-alarm blaze at 11 Temple St. in Beacon Hill at 12:10 a.m. Sunday, officials said.

Between 20 and 25 people were displaced by the Beacon Hill fire. The apartment building had an estimated $750,000 in damage, according to the Boston Fire Department.

Fire Commissioner Joe Finn commended firefighters for their “fast, aggressive’’ action to control the fires.

Adam Sennott can be reached at adam.sennott@Globe.com.