Definition of sheaf using equalizer Wikipedia give sheaf property using equalizer diagram by saying sheaf property means for any open cover $\{U_i\}$ of $U$
$$F(U) \rightarrow \prod_{i} F(U_i) {{{} \atop \longrightarrow}\atop{\longrightarrow \atop {}}} \prod_{i, j} F(U_i \cap U_j)$$
is an equalizer. What means two arrows? If $i = 1,2,3$ then what means
$$F(U) \rightarrow F(U_1) \times F(U_2) \times F(U_3) {{{} \atop \longrightarrow}\atop{\longrightarrow \atop {}}} F(U_1 \cap U_2) \times F(U_2 \cap U_3) \times F(U_1 \cap U_3)?$$
 A: Given maps $f:X\to Y$ and $g:X\to Y$, a natural way of writing them together in the same diagram would be like this:
$$X{{f\atop {\Large\longrightarrow}}\atop{{\Large\longrightarrow}\atop g}}Y$$
A map $\mathrm{eq}:E\to X$ is an equalizer (Wikipedia) of the maps $f$ and $g$ if it is final in the category of maps to $X$ that equalize $f$ and $g$. Depicting it together with the maps $f$ and $g$, we say that 
$$E\xrightarrow{\;\mathrm{eq}\;} X{{f\atop {\Large\longrightarrow}}\atop{{\Large\longrightarrow}\atop g}}Y$$
is an equalizer diagram.

Let $X$ be a topological space, let $F$ be a presheaf on $X$, and let $\{U_i\}_{i\in I}$ be an open cover of $X$. The presheaf $F$ comes with restriction maps $\mathrm{res}_{V}^{U}:F(U)\to F(V)$ for each pair of open sets $U,V$ with $V\subseteq U$. In particular, for each pair of $i,j\in I$, we have restriction maps $$\large\mathrm{res}_{U_i\cap U_j}^{U_i}:F(U_i)\to F(U_i\cap U_j)$$ and  $$\large\mathrm{res}_{U_i\cap U_j}^{U_j}:F(U_j)\to F(U_i\cap U_j).$$
By definition, for each $i\in I$, the product (Wikipedia) $\prod_{i\in I}F(U_i)$ comes with a projection map 
$$p_i:\prod_{i\in I}F(U_i)\longrightarrow F(U_i).$$ Moreover, $F(U_i)$ comes with a restriction map $\mathrm{res}_{U_i\cap U_j}^{U_i}:F(U_i)\to F(U_i\cap U_j)$ for each $j\in I$. Composing them, we have for each pair of $i,j\in I$, a map $a_{i,j}$, as follows:
$$\underbrace{(\mathrm{res}_{U_i\cap U_j}^{U_i}\circ p_i)}_{\Large a_{i,j}}:\prod_{i\in I}F(U_i)\longrightarrow F(U_i\cap U_j)$$
By the definition of a product, these $a_{i,j}$'s induce a map (let's call it $\alpha$) from $\prod_{i\in I}F(U_i)$ to the product of all of the $F(U_i\cap U_j)$'s together:
$$\alpha:\prod_{i\in I}F(U_i)\longrightarrow \prod_{i,j\in I}F(U_i\cap U_j).$$
Similarly, for each pair of $i,j\in I$, we have a map $b_{i,j}$:
$$\underbrace{(\mathrm{res}_{U_i\cap U_j}^{U_j}\circ p_j)}_{\Large b_{i,j}}:\prod_{i\in I}F(U_i)\longrightarrow F(U_i\cap U_j)$$
which come together to form a single map
$$\beta:\prod_{i\in I}F(U_i)\longrightarrow \prod_{i,j\in I}F(U_i\cap U_j).$$
These maps $\alpha$ and $\beta$ are the top and bottom arrows in the diagram
$$F(U) \longrightarrow \prod_{i\in I} F(U_i) {{{}\atop {\Large\longrightarrow}}\atop{{\Large\longrightarrow}\atop {}}}\prod_{i, j\in I} F(U_i \cap U_j)$$
We say that $F$ is a sheaf (Wikipedia) if, for any open cover $\{U_i\}_{i\in I}$ of our space $X$, this diagram is an equalizer diagram.

Note that $\alpha$ and $\beta$ are not the same map; they take different "paths". Let's trace out what happens: the top-right path is $a_{i,j}$, and the bottom-left path is $b_{i,j}$.  $\require{AMScd}$
$$\require{AMScd}
\begin{CD}
\prod_{i\in I}F(U_i) @>{\Large p_i}>> F(U_i);\\
@V{\Large p_j}VV @VV{\Large\mathrm{res}_{U_i\cap U_j}^{U_i}}V \\
F(U_j) @>>{\Large\mathrm{res}_{U_i\cap U_j}^{U_j}}> F(U_i\cap U_j);
\end{CD}$$
The two maps $\alpha$ and $\beta$ to the product $\prod_{i,j\in I}F(U_i\cap U_j)$ agree on a specific element $(s_i)_{i\in I}$ of the product $\prod_{i\in I}F(U_i)$ if and only if $a_{i,j}$ and $b_{i,j}$ agree on it for all pairs of $i,j\in I$. But $a_{i,j}$ and $b_{i,j}$ are clearly going to be different maps in general, so this is not a trivial condition.
