Find the number of ways to select 2n balls from n identical blue balls, n identical red balls and n identical white balls, where n $\in$ $\mathbb N$ Q: Find the number of ways to select 2n balls from n identical blue balls, n identical red balls and n identical white balls, where n $\in$ $\mathbb N$.
My working:
$(x+x^2+x^3+...)^3$
$=x^3(1+x+x^2+...)^3$
$= x^3 \sum_{r=0}^\infty \begin{pmatrix} {r+3-1}\\{r}\end{pmatrix}x^r$
$= \sum_{r=0}^\infty \begin{pmatrix} {r+2}\\{r}\end{pmatrix}x^{3+r}$
Hence the number of ways is $\begin{pmatrix} {2n-1}\\{2n-3}\end{pmatrix}$=$\begin{pmatrix} {2n-1}\\{2}\end{pmatrix}$. 
However, the actual answer is $\begin{pmatrix} {2n+2}\\{2}\end{pmatrix}$$-3$$\begin{pmatrix} {n+1}\\{2}\end{pmatrix}$. Did i go wrong somewhere in my proof? Or is it some conceptual understanding gone wrong? Thanks!

I think it's easiest to count this directly, without any fancy tricks.  We actually end up with a simpler formula: $$\binom{n+2}{2}$$ or, equivalently, $$\frac{(n+1)(n+2)}{2}$$ (which is equal to the longer formula in @Muralidharan's answer).
Here's how it works:
The number $r$ of red balls is in the range from $0$ to $n,$ and the number $b$ of blue balls is also in the range from $0$ to $n.$
Given values for $r$ and $b,$ there's at most one possible value for the number of white balls: $w = 2n-r-b.$  But this potential number of white balls also has to be in the range from $0$ to $n.$  If $2n-r-b$ is in the range from $0$ to $n,$ then $\langle r, b, 2n-r-b\rangle$ is a triple we have to count.  If $2n-r-b$ is not in that range, then we should skip that triple and not count it.
Since each ordered pair $\langle r, b\rangle$ has at most one corresponding $w,$ this is the same as counting the number of ordered pairs $\langle r, b\rangle$ such that $0 \le r \le n, 0 \le b \le n,$ and $0 \le 2n-r-b \le n.$
So our count is as follows, using the notation $\#X$ for the cardinality of a set $X\!\!:$
\begin{align}
&\#\lbrace \langle r, b\rangle \mid 0 \le r \le n \;\land\; 0 \le b \le n \;\land\; 0 \le 2n-r-b \le n\rbrace
\\= &\#\lbrace \langle r, b\rangle \mid 0 \le r \le n \;\land\; 0 \le b \le n \;\land\; r + b \le 2n \;\land\; n \le r+b\rbrace
\\= &\#\lbrace \langle r, b\rangle \mid 0 \le r \le n \;\land\; 0 \le b \le n \;\land\; n-r \le b \le 2n-r\rbrace
\\= &\#\lbrace \langle r, b\rangle \mid 0 \le r \le n \;\land\; \max(0,n-r) \le b \le \min(n,2n-r)\rbrace.
\end{align}
For any $r \le n,$ we have $\max(0,n-r)=n-r$ and $\min(n,2n-r)=n,$ so the count above equals
\begin{align}
&\#\lbrace \langle r, b\rangle \mid 0 \le r \le n \;\land\; n-r \le b \le n\rbrace.
\end{align}
You can see that for each value of $r$ from $0$ to $n,$ we have $r+1$ possible values of $b$, namely $n-0, n-1, n-2, \dots, n-r.$
It follows that the total count equals
\begin{align}
\sum_{r=0}^n (r+1) &= \sum_{k=1}^{n+1} k
\\&= \frac{(n+1)(n+2)}{2},
\end{align}
and this equals $$\binom{n+2}{2}$$ if you want to phrase the answer as a binomial coefficient.
