We've had unstable aircraft with computer-controlled control surfaces for over 40 years, the inherent inertia coupling tendencies will be controlled by that same philosophy.
The X-3 was designed in the late 40s/early 50s and hence couldn't benefit from small lightweight digital computers because they were not developed until 15 or more years later.
Just read the remainder of the X-3 Wikipedia article. Quite sobering. 260kt take-off speed and +/-7g roll-coupled pitch oscillations on the flight where Joe Walker nearly lost control.