3 letter monograms:

• First name initial, last name initial and middle name initial is the most common format, with the last name appearing larger than the other two intials.   

Martha Susan King would look like MKS.

• Alternatively, the initials can all be the same size, monogrammed in order of first, middle, and last name.     

Timothy Richard Smith would look like TRS.

A married couple:

A.  Modern Monogram:

• Woman's First Name,  Married Last Name,  Husband's First Name (with last name initial in larger size). 
• So,  Martha Susan King  married to Timothy Richard Smith  would look like  MST.

B.  Traditional Monogram:  

• Woman's First Name, Married Last Name, Woman's Maiden Name (with last name initial in larger size). 
• Martha Susan King married to Joseph William Reid would look like MRK