It is not clear if your column is a character column or a date-formatted numeric column. Here's a way to get the 3-letter month names from JMP without building a 12-item lookup table:
x=informat("01/12/2017","d/m/y"); // use this if the data is character, creates a date-time value
y=format(x,"ddMonyyyy"); // use this to get a string from a date time: "01Dec2017"
z=substr(y,3,3); // use this to pick out the month: "Dec"
You can also ask the date time value for the day, month, and year parts as numbers: month(x), day(x), year(x).