I am trying to read GJ部 and I've come across this:
紫音さんの細い指が喉元で動く。 三人の手が三人とも違っていることに、軽い驚きがあった。
I think this sentence means something like "Shion-san's thin fingers moves around his neck. He was slightly surprised at how different each of the three girls' hands feel", but I don't really understand what ことに is doing in this case.
Context: This guy couldn't tie his tie so 3 girls offered to help him, the first two also tied it wrong and while the third girl was trying to tie it the narrator said this.