The short answer is yes. Pitch, roll and heading will be given correctly in full 3D orientation.

The longer answer is yes, as long as the main horizontal axis (X by default) is not pointing vertically up.

The explanation for this comes from Euler angles and the phenomenon of gimbal lock. Gimbal lock occurs when the main horizontal axis (i.e. where we define heading, X by default) and vertical (i.e. where we define tilt = 0, Z by default) align. It is in this case where heading is no longer readable. For a profiling instrument this really isn't a drawback since we almost always want to measure either consistently pointing vertically or sideways and not rotating in 3D.