For a monogram the family name initial goes in the center and larger,bolder and is flanked by the first and middle initial on either side, these are smaller.
Such as: J S A but with the S maybe double the size.
listing them as JAS would be to just list the initials of the person. Letters equal size and weight.
Some people just want their "initials" on a gun. Could be 2, first /last or 3, first,middle, last.
Others are very specific and want them in a "monogram"
I have done them both ways depending on the desire of the customer.
Hope that helps
Mark