Why is 3D stress tensor acting only on three surfaces? I'm trying to learn about the stress tensor (in 3D)

The tensors are said to have directions (the first subindex $i$ in $\sigma_{ij}$) and specify the surface upon which they act (the second subindex $j$ in $\sigma_{ij}$).
What confuses me is why is it defined only to act on three surfaces, even when the cube has six surfaces?
 A: That diagram shows only that set of three arrows for clarity: it is the smallest set that displays all the information necessary, but adding arrows to all six faces would make the diagram too crowded.
In general, if you have a material with a given stress tensor $\sigma_{ij}$, and you look at one small internal surface patch of area $A$ and normal vector $\vec n$, then the force $\vec F$ transmitted by the material's stress across that surface patch is given by
$$F_i=\sum_j \sigma_{ij}n_j A.$$
In particular, for the three faces shown with $\vec n=\hat e_k, \ k=x,y,z$, you get a force with $i$th component $\sigma_{ik}$ acting on the face with normal $\hat e_k$. For the back faces, $\hat n=-\hat e_k$ changes direction, so the forces change direction, but they're obviously still there.
(It's a good exercise at this point to draw those arrows for one face and its opposite. You should get normal arrows pointing away from each other, either pushing in (pressure) or pulling out (tension). The tangential arrows ($\sigma_{yx}$ and $\sigma_{zx}$, say) should point away from each other in a shearing motion. The net result should be deformations of the unit cube, but no net force.)
A: We do consider other 3 faces, as we take into account (Sigma x), it means other opposite face of it is also taken but written only once which means the magnitude is same but direction is opposite, i.e. you will see 9 components but still there are 18 components, we only write 9 since other 9 will have same magnitude. So, think twice is it worthy to write twice ? answer is NO as if you know the magnitude, you can apply it on the element, if you are still confuse then think we take element under equilbrium condition.
