That would be "three and one-hundred thousandths". You cannot reduce the number given, 3.100, to a less exact number 3.1 ("three and one tenth"), without changing the significance of the exactness of the value.
How Do You Write 3.100 In Word Form?
Three and one tenth
3 and 1 tenth
Five and two hundred sixty-seven thousand
Three Point One
not Three Point One Zero Zero as the 0's on the right after a . Don't count
not Three Point One Zero Zero as the 0's on the right after a . Don't count