Pointwise convergence of averages along cubes

Let $(X,\mathcal{B},\mu ,T)$ be a measure preserving system. We prove the pointwise convergence of the averages $$\frac{1}{N^2}\sum _{n,m=0}^{N-1}{f}_1(T^{n}x)f_2(T^{m}x)f_3(T^{n+m}x)$$ and of similar averages with seven bounded functions.