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.

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