Are you looking for Justin's parents? The 1900 census gives his birth date as March 1870, and his mother's birth place as Maryland, so I suspect he is the "Ulysses" Cross, born March 1870, living in Islesboro in 1870 with George Cross (born in Maine) and Frances Cross (born in Maryland). The family had moved to Rockland by 1880, and he may have been the son "George," aged 11 years, then living with them.
Yeah, that is interesting. Hard to tell if it's due to errors in the censuses or actual name changes. Many people switched between using their first and middle names when they became adults, but three different names over 30 years is unusual. And Justin's middle initial was apparently "L.," which rules out George or Ulysses as a middle name.