How do we take (something) $(mod~(3, x^{n-1}+x^{n-2}+...+1))$ and what is a normative canonical representative of a polynomial?

Question

I have two questions in the following explanation:

"Consider the quotient ring $S/3=Z_3[x]/(3,x^{n-1}+x^{n-2}+...+1)$. The canonical S/3-representative of $a\in Z[x]$ is the unique polynomial $b\in Z[x]$ of degree at most $n-2$ with coefficients in $\{-1,0,1\}$ such that;

$a\equiv b $ $(mod~(3, x^{n-1}+x^{n-2}+...+1))$. We write $\underline S3(a)$ for the canonical S/3 -representative of $a $. We write $S3(a)$ when the choice of representative is not normative".

Question 1: How do we take modulo of : (something) $(mod~(3, x^{n-1}+x^{n-2}+...+1))$ ? Do we first take (something) mod 3, then take $(mod~( x^{n-1}+x^{n-2}+...+1))$ of it afterwards?

Question 2: What does normative mean in the context above?