It's always fascinating reading of past exploits of soldiers. As an ex infantry soldier myself that fought in a small but vicious war in the Falklands in 1982, I can have a little sympathy for these three men. War is by its very nature is not very nice and sometimes you are asked to do something unspeakable by your officers.
On the face of it what they did if they did it in 2020 would not raise more than an eyebrow, they certainly would not face a firing squad. However, we are talking about an era when millions of people in Europe had been slaughtered by the Nazis
It is a war crime to dress in the uniform of your enemy and the penalty would have been death just as it would have been if they had been dressed as civilians, in that situation they would have been considered spies. They could in that situation pleaded innocence saying they were German deserters, it would have been touch and go but I would say if they all stuck to the same story and were smart enough not to show patriotism to Hitler they would have lived. But to be caught behind enemy lines in the uniform of your enemy in those days the outcome was inevitable.
In many ways, we have to admire their bravery, it took some guts to go through the American lines with no prospect of getting rescued if it went tits up which it did, it also took guts to face that firing party as well and show no fear and to sing German patriotic songs as they were being tied to the posts, which makes me think they were fanatical German Nazis.