Matrix representation
normal component
Norm = Va P = (Va × P) * P/P²
This is derived on this page.
parallel component
Proj = Va  P = Va • P * P/P²
This is derived on this page.
Reflection matrix
Given the inversion I'll add the terms instead of subtracting them to give the reflection result:
p > 1 / (Px² + Py² + Pz²)* 

[p] 
Note that this matrix is symmetrical about the leading diagonal, unlike the rotation matrix, which is the sum of a symmetric and skew symmetric part.
Simple cases
In order to check the above lets take the simple cases where the point is reflected in the various axis:
Reflection in yz
1  0  0 
0  1  0 
0  0  1 
Reflection in xz
1  0  0 
0  1  0 
0  0  1 
Reflection in xy
1  0  0 
0  1  0 
0  0  1 
Determinant and eigen values
Another check is that the determinant of reflection matrix is 1
ExampleAs an example we want to reflect the point (1,0,0) in a plane at 30 degrees.
where Px,Py,Pz is the normal to the mirror which is: (0.5,0.866,0) and P1 is the initial point which is (1,0,0) substituting these values gives:
multiplying to vector by the matrix gives:
