Binomial expansion theorem is well established, and technique to determine rth term for any binomial expansion function is also known. How to find precisely rth term in a trinomial expansion function: more specifically how to find 2nd,100th and 1600th term of (a-b-c)^125
multinomial theorem to find any given term in such an expansion.
However, there is no natural total ordering of the terms in a multinomial expansion. You have to specify how you are going to order the terms before it makes sense to talk about the 2nd, 100th or 1600th term of (a-b-c)^125.