From Longman Dictionary of Contemporary Englishthird-degree murder/burglary/assault etcthird-degree murder/burglary/assault etcAmerican English murder etc that is considered by a court to be the least serious of three different kinds → third-degree
Examples from the Corpus
third-degree murder/burglary/assault etc• Butch finally pleaded guilty to third-degree murder and was sentenced to 10 to 20 years. 