Why 3 and not 6 in U = 3/2RT from the equation for the internal energy of an ideal gas?

Internal energy of an ideal monatomic gas is


$$U=\frac{3}{2}RT.\tag{1}$$


While I understand the derivation, I do not understand why we multiply by $3$ rather than by $6.$ According to [Khan Academy video](https://www.youtube.com/watch?v=qSFY7GKhSRs) the force of gas molecules hitting *one* of the six faces of the wall is


$$pV=Nmv\_x^2.\tag{2}$$


Why don't we multiply by $2$ after we multiply by $3$ to account for the pressure arising in the $y$ and $x$ directions as well to account for the force applied to all six faces of the theoretical box?


