To find the sum of three consecutive integers where the least integer is \( n \), we can express the three integers as \( n \), \( n + 1 \), and \( n + 2 \).
Therefore, the equation that represents the sum of these three integers is:
\[ n + (n + 1) + (n + 2) = 90 \]
Simplifying this, we get:
\[ n + n + 1 + n + 2 = 90 \]
\[ 3n + 3 = 90 \]
So the correct equation from the options given is:
D. n + (n + 1) + (n + 2) = 90.