Only A3 has a "proper" LU decomposition (without pivoting). Hopefully this much you had no problem with.

As far as finding the decomposition, there are a number of algorithms you can find online. However, for 3x3 it is easy enough to solve on your own.

\(\displaystyle A=\begin{bmatrix} 1 & 0 & 0 \\ l_{2,1} & 1 & 0 \\ l_{3,1} & l_{3,2} & 1\end{bmatrix} \begin{bmatrix} u_{1,1} & u_{1,2} & u_{1,3} \\ 0 & u_{2,2} & u_{2,3} \\ 0 & 0 & u_{3,3} \end{bmatrix}\)

Nine equations, nine unknowns.