Are there some site that I can convert the following set of rotations, of a 3D picture, i.e.

Rot Axe Y → Rot Axe X → Rot Axe Z → Rot Axe X → Rot Axe Y (or any other set of rotations)

in

Rot Axe X → Rot Axe Y → Rot Axe Z (in any sequence)

I have the intuition that can be done through matrices manipulation. But perhaps there are available some site that can input the angles and get the output angles. Or some script already done. Or some Matlab instruction. In last case you can give me the theoretical concerning that.

Notice that we named the angles yaw, pitch and roll since whether they actually correspond to rotY, rotX and rotZ depends on your axes conventions (e.g. X is the first axis, right- or left handed, etc.).

However, the solution above can exhibit numerical instability whereas the header-olny library Eigen’s implementation (Eigen: Main Page) is gimbal-lock-proof:

#include <Eigen/Geometry>
(...)
Matrix3f R = ...; //multiply all your input rotations here.
Vector3f angles = R.eulerAngles(2, 1, 0).reverse();