Fourteen colonels, 15 lieutenant colonels, a major and three captains were also on the list of 721 military killed between March 18 and November 24.
The letter was sent to the United Nations by Syria’s UN mission in December but an annex giving the names and ranks of the dead was only released on Tuesday.
The three brigadier generals killed include Adnan Zaydan Dib, a top officer in the Syrian special forces who was wounded in Homs in August and died in hospital the following month, according to the letter.
Since the list was drawn up, one other senior general was gunned down in Damascus in February, official media said. Dissident soldiers killed another general in Hama in January after refusing to carry out his orders, according to Syrian activists.
The Syrian government said last week that 2,088 soldiers and security agents and 478 police have now been killed since mid-March.