The businessman who was viciously beaten by the Swiss teenagers in Munich last year was the first victim to testify about the spree of attacks.
The three youth are charged with attempted murder and grievous bodily harm.
The 46-year-old had his face “completely shattered”, and is undergoing a series of operations to reconstruct it. His eyesight and hearing have also been severely weakened by the attack.
Witnesses have told investigators that two of the accused delivered heavy kicks to the man’s head after he had been knocked to the ground.
The three 16-year-olds from canton Zurich travelled to the German city as part of a class trip last June. They had been drinking before heading for the city centre.
They are accused of having randomly attacked five people over the course of 30 minutes: first, beating three unemployed people before moving on to the businessman and later a 27-year-old Bulgarian student.
Only one of the of the five victims was able to identify the youths, all of whom have previous convictions.
swissinfo.ch and agencies