Alright, so I have Velocities x y and z reflecting off of a flat surface. It is a perfect reflection (no velocity is lost). I have 3 points on the flat surface (each with x,y,z.. a triangle) which i took the Normal angle of using the vector of the 3 points.
There's really two parts to this problem, solving for the new Vxy and solving for the new Vz.

Found Variables:
fx, fy, fz (Normal vector from the surface of the triangle)
d = pythagorean using fx and fy
Vt (Velocity of the particle)
Vx, Vy, Vz (Initial velocities of the particle)
AngleXY = Atan(fy/fx) (XY Angle of the normal vector = XY Normal Angle)
AngleZ = Atan(fz/d) (Z Angle of the normal vector = Z Normal Angle)
VelAngleXY (XY Angle of the intial Velocities, found through atan)
VelAngleZ (Z Angle of the initial Velocities, found through atan)

My Attempt:

Solving for Vx, Vy
FinalAngleXY = (-XYAngle)-(VelAngleXY+AngleXY)
Vd = Pythag using Vt and Vz to find adjacent component
FinalVx = Vd*Cos(FinalAngleXY)
FinalVy = Vd*Sin(FinalAngleXY)

Solving for Vz
FinalAngleZ = -AngleZ-(VelAngleZ+AngleZ)
FinalVz = Vt*Sin(FinalAngleZ)

What I found:
When the particle hits a slope, no matter the slopes direction it will reverse the particle's direction. Example: You throw a ball down a hill, it bounces back up instead of down (mainly due to the new XY velocities, but the new Z velocity is off too)

Any help is much appreciated! Been stuck on this for awhile now