Is 3 crew cockpit safer than 2 crew?
No aircraft ever had more than two pilots (on duty at the same time).

The additional flight crew members were flight engineer, navigator and radio operator.


Dedicated radio operators were only needed in the early days when the radio was primitive and required some care and experience to tune properly. As it improved, operating radio was merged into navigator or flight engineer job and eventually became simple enough for pilots to do themselves.
Navigators were used on long-distance flights especially over water. They kept track of the progress on the map and corrected the position using celestial navigation and some landmarks where available. With advent of inertial navigation systems and long range radio this simplified enough that dedicated crew member was not needed any more.
Flight engineer was most common and remained longest. Engines had many gauges that needed monitoring and controls that had to be correctly set to start them and adjusted to keep them running efficiently. And while turbine engines are simpler to manage than piston ones, the addition of more electric and hydraulic system meant that there was still a lot to set up and monitor. But now this job is done by a computer that monitors all the parameters and displays specific messages when something fails including instructions what needs to be checked and done for each problem.


In the meantime, the improvements in autoflight systems freed more time for the pilots in which they can now handle, with help of the electronics, the tasks that before needed a dedicated crew member.

Due to these technological advances allowing for the removal of these crew members, overall flight safety has not been compromised. It is likely that safety has actually improved by reducing the probability of human error.