Triple Summation Of Product Of Binomial Coefficients

Question

$ \sum\sum\sum\binom{n}{i}\binom{n}{j}\binom{n}{k} $ Also it is given $0\leq i\leq j\leq k\leq n$. I can not think of the solution. I can understand the question if it was $ \sum\sum\binom{n}{i}\binom{n}{j} $ and $0\leq i\leq j\leq n$. Please provide a detailed solution if possible.