3. As shown in Figure 1, in a $5 \times 5$ square grid with a side length of 1, there is a lattice point $\triangle A B C$. Find a lattice point $\triangle D E F$ in the grid that is similar to $\triangle A B C$. The maximum area of $\triangle D E F$ is ( ).
(A) 2
(B) 2.5
(C) 3
(D) 3.5
3. B.

Notice that, $AB=1, BC=\sqrt{5}, AC=\sqrt{10}$, and the diagonal of the square is $5\sqrt{2}$. Since $5\sqrt{2} \div \sqrt{10} = \sqrt{5}$, we can find a triangle with side lengths $\sqrt{5}, 5, 5\sqrt{2}$, as shown in Figure 7, $\triangle DEF$ is the required triangle, with a similarity ratio of $\sqrt{5}$.
Given $S_{\triangle ABC}=\frac{1}{2}$, we know $S_{\triangle DEF}=(\sqrt{5})^{2} S_{\triangle ABC}=2.5$.