In how many ways can $2t+1$ identical balls be placed in $3$ boxes so that any two boxes together will contain more balls than the third? 
In how many ways can $2t+1$ identical balls be placed in $3$ boxes so that any two boxes together will contain more balls than the third? 

I think we have to use multinomial theorem, but I cannot frame the expression!

There must be at least $1$ ball in the first box; why? On the other hand, there cannot be more than $t$ balls in the first box; why? 
Say there are $n$ balls in the first box, where $1\le n\le t$. If there are $m$ balls in the second box, what are the possible values for $m$? We have to be sure that 
$$m+n>(2t+1)-(m+n)$$
and that 
$$n+\big((2t+1)-(m+n)\big)>m\;.$$
Solving these for $m$, we see that we have to be sure that
$$t-n+\frac12<m<t+\frac12\;.$$
Since $t,n$, and $m$ are integers, this is equivalent to $t-n+1\le m\le t$. Any ordered pair $\langle n,m\rangle$ satisfying the conditions $1\le n\le t$ and $t-n+1\le m\le t$ corresponds to a possible distribution of balls to the first two boxes, with the third box getting the remaining $2t-n-m$ balls, and each possible distribution of balls yields one of these ordered pairs.
You can count these pairs by summing the possibilities for different values of $n$, as in the answer by ddsLeonardo, or you can count them all at once by a more combinatorial argument, as follows.
Line up $t$ of the balls. There are $t-1$ slots between adjacent balls and one slot on each end of the line, for a total of $t+1$ open slots. Insert markers into any two distinct slots; this can be done in $\binom{t+1}2$ ways. Let $m$ be the number of balls to the right of the lefthand marker and $n$ the number of balls to the left of the righthand marker. Clearly $n$ can be any integer from $1$ through $t$, and $m$ can then be any integer from $t-n+1$ through $t$. Thus, each placement of the two markers corresponds to a possible ordered pair $\langle n,m\rangle$ of the numbers of balls in the first and second boxes, and vice versa. Since there are $\binom{t+1}2$ ways to place the markers, there are $\binom{t+1}2$ possible distributions of the balls.
