Which family of boxing brothers, had three of them get in the ring with three heavyweight champions among others?
Yes sir! Victor fought Johnson in a no dec 6 rounder ,[Johnson 211lbs Victor 198lbs ,] Victor was a late sub for Denver Ed Martin. Vic also went in with a young Jess Willard, who beat the crap out of him making him quit in 4rds of a gym fight. Fred, who fought as Fred Mckay sparred with Willard too, he was kod by Carl Morris, Boer Rodel, and Gunboat Smith among others.. Brother Arthur fought 3 exhibitions with Bob Fitzsimmons in 1910.. Brother Leon all 6' 8" of him was kod by Fireman Jim Flynn.
To be honest I didn't know Victor McLaglen had any brothers but the name McLaglen is on the photo you posted.
McLaglen had six brothers and a sister, six of the brothers enlisted in the Army at the start of WW1 , five returned but Fred was killed.