Question 1019520
Consider the denominators present in every rational expression, namely {{{n^2}}}, n, and 3n.  To find the LCD, find the highest power of any distinct factors appearing, and then multiply them together.
One factor is the number 3, whose highest power is 1. (So {{{3^1}}}, or 3, appears in the LCD).  The factors n and {{{n^2}}} also appear, both of them powers of the same base n.  So choose {{{n^2}}}, because it is the highest power of n appearing. 
The LCD is then {{{3n^2}}}.