A fire in Mattapan early Saturday morning injured three firefighters and displaced six residents, according to a Boston Fire Department spokesman. The firefighters were taken to a Boston hospital for a knee burn, heat exhaustion, and a back injury, said Boston Fire spokesman Steve MacDonald. No other injuries were reported. The three-alarm fire at 15-17 Evelyn St. was reported about 3:36 a.m. and was mostly contained within 45 minutes, MacDonald said. The cause is under investigation. Four adults and two children were displaced from the two-family home. Damage was estimated at $500,000, MacDonald said. The fire melted the siding on a neighboring house on Evelyn Street, causing an estimated $50,000 in damage.