matrix multiplication
myMatrix.LoadIdentity();
//last rotation is pitch
myMatrix.SetPitch(-90);// myMatrix = -90 deg pitch
//second rotation is roll
myMatrix.Mult(rollMatrix);// rollMatrix * myMatrix => myMatrix
//first rotation is yaw
myMatrix.Mult(yawMatrix);// yawMatrix * myMatrix => myMatrix
final result in myMatrix is yaw then roll then pitch