He told the state court that he and fellow members of the SS Silbertanne (Silver Pine) death squad had been informed by their superiors that the men were to be killed in retaliation for attacks by the resistance.
"I knew that if I didn't carry out my orders I would be breaking my oath and would be shot myself," he said.
"At no time in 1944 did I act with the feeling that I was committing a crime," he added. "Today, after 65 years, I naturally see things from a different perspective."
But the presiding judge, Gerd Nohl, told the court that all three killings had been carried out "on a totally random basis" and constituted murder.
"These were murders that could hardly be outdone in terms of baseness and cowardice - beyond the respectability of any soldier."
Members of the death squad had worn "civilian clothes, rain coats, and carried out the crimes either early in the morning or late in the evening", and the risk to Boere when he shot the three men had been "zero", he added.