This link on the internet states 3/12 would be 3 months of 12: http://usa.ipums.org/usa/voliii/items1880.shtml
The Census of 1880

Go Back to Enumeration Forms Index

Schedule 1. Free Inhabitants in _________, in the County of ________, State of _________, enumerated by me, on the _____ day of ___________, 1880.

_____________________, Enumerator.
Enumeration district No. ___________.

In cities: Name of street; house number
1.Dwelling houses numbered in order of visitation.
2.Families numbered in order of visitation.
3.The name of every person whose place of abode on the 1st day of June, 1880, was in this family.

Personal description:
4.Color - White, W; black, B; Mulatto, Mu; Chinese, C; Indian, I.
5.Sex - Males (M), females (F)
6.Age at last birthday prior to June 1, 1880. If under 1 year, give months in fractions, thus, 3/12.
7.If born within the census year, give the month.
8.Relationship of each person to the head of this family - whether wife, son, daughter, servant, boarder, or other.