What bound can we get using $k$-th moment inequality under 3-wise independence? Let $X_1,\dots, X_n$ be 0-1 random variables, which are $3$-wise independent. We want to give a upper bound to $\Pr(|\Sigma_iX_i-\mu|\geq t)$. Can we get better bound than $\Theta\left(\frac{1}t\right)$ derived by Chebyshev's inequality?

If we get a little more information, say $4$-wise independence, then we can use the $4$-th moment to get a $\Theta\left(\frac{1}{t^2}\right)$ bound. Let $Y_i = X_i - \mu$ , $Y = \Sigma_iY_i$, 
$$\Pr(|Y|\geq t) = \Pr(Y^4\geq t^4) \leq\frac{\mathbb{E}(Y^4)}{t^4}=\frac{t\mathbb{E}(Y_1^4)+3\cdot t\cdot (t-1)\mathbb{E}(Y_1^2)}{t^4}=\Theta\left(\frac{1}{t^2}\right).$$

If we only have $3$-wise independence, the numerator in the RHS will be $$t\mathbb{E}(Y_1^4)+3\cdot t\cdot (t-1)\mathbb{E}(Y_1^2)+24\cdot\sum_{i&lt;j&lt;k&lt;l}\mathbb{E}(Y_iY_jY_kY_l).$$ I don't know how to deal with the summation.

I guess the answer may lie around $\Theta\left(\frac{1}{t^{1.5}}\right)$?
I guess that the number of random variables $t$ and the threshold $t$ are different parameters, as otherwise $\Pr[|Y| \geq t] = 0$.

Let $a_1, \dots, a_k, b_1, \dots, b_k\in_U \{\pm 1\}$ be iid random variables sampled uniformly at random from $\{\pm 1\}$ and $n=2^k$. Consider random variables $W_1,\dots, W_n$ of the form $c_1 \cdot c_2\cdot \dots \cdot c_k$ where each $c_i$ is either equal to $a_i$ or $b_i$ (e.g., one of random variables $W_i$ equals $a_1b_2b_3a_4b_5  \dots a_k$). We defined $n$ random variables $W_i$; note that they are $3$-wise independent. 

Let $X_i = (W_i+1)/2$.
Then $X_i\in\{0,1\}$ and ${\mathbb E}[X_i] = 1/2$. Let $t=n/2$. We have, 
$$\Pr(|\sum X_i - n/2| \geq t) = \Pr(|\sum W_i| \geq n) = \Pr(|\prod_{i=1}^k(a_i+b_i)| \geq n) = \Pr(a_i+b_i\neq 0 \text{ for every }i) = \frac{1}{2^k} = \frac{1}{2t}.$$
Here, we used that $\prod_{i=1}^k(a_i+b_i)$ equals $0$ if $a_i+b_i=0$ for some $i$, and equals $-n$ or $n$, otherwise.

Answer: in general, we cannot obtain a stronger upper bound than $O(1/t)$.

Update: here is an explanation why $W_1,\dots, W_n$ and consequently $X_1,\dots, X_n$ are 3-wise independent. Each of the words $W_i$ is encoded by a word of length $k$: the $r$-th letter in the word is $a$ if $c_r=a_r$ in the definition of $W_i$, and $b$, if $c_r=b_r$. Consider three random variables $W_i$, $W_j$ and $W_k$. Consider the corresponding words; denote them by $I$, $J$ and $K$. Let $r_1$ be the first position where they differ. Without loss of generality $I_{r_1} = a$, $J_{r_1} = b$, and $K_{r_1} = b$. Then let $r_2$ be the first position where $J$ and $K$ differ. WLOG, $J_{r_2} = a$ and $K_{r_2} = b$. Write $W_i = a_{r_1} \cdot W'_i$, $W_j = a_{r_2}\cdot W_j'$ and $W_k = b_{r_2}\cdot W_k'$. Note that $W_j$ and $W_k$ are independent since for $x,y\in\{\pm 1\}$, we have
$$\Pr(W_j = x, W_k = y) = \Pr(a_{r_2} = (x/W_j'), b_{r_2} = (y/W_k')) = 1/4.$$
Now we prove that $W_i$, $W_j$, and $W_k$ are independent. For  $x,y,z\in\{\pm 1\}$, we have
$$\Pr(W_i = x, W_j = y, W_k = z) = \frac{1}{4}\Pr(W_i = x| W_j = y, W_k = z) = 
\frac{1}{4}\cdot \Pr(a_{r_1} = (x/W'_i)| W_j = y, W_k = z) =  \frac{1}{4}\cdot \frac{1}{2} = \frac{1}{8}.$$