Simulink integrator initial conditions - activate 1 or a 3vector upon a reset

I can solve this problem by using separate integrators for each of 3 degrees of freedom, but I hope to achieve this using integrators with 3vector inputs/outputs. I want to reverse a single components of a velocity 3vector upon a collision being detected. This is for a gas molecular dynamics visualization using simulink and simulation 3D animation. Consider a single molecule moving inside a rectangular box. When the molecule hits a side of the box, the velocity component that is perpendicular to that face is reversed while the remaining 2 veloctiy components remain unchanged. The integrator allows me the use a 3vector for saturation limits, but I cannot determine whether the resulting reset can be configured to indicate which saturation limit caused the reset; thus, allowing me to apply a single (out of 3) initial condition to reflect the reversed velocity.

As I said, I can achieve this by using integrators for each of the axes. I'm trying to simplify the simulink model to make it easier to explain and visualize.