Probably a few more things to consider.
A read through
FAA AC 120-42A is worth the time ......
The basic idea is to achieve something comparable to four-motor reliability. It is not just a question of number of motors but overall system redundancy and reliability.
(a) each airframe/engine combination is considered separately
(b) individual critical systems are considered. So, for instance, a particular widget may need particular SBs etc to be incorporated for the widget to qualify for use in ETOPs operations by virtue of different reliability characteristics. This sort of thing can make configuration status tracking a bit of a nightmare for the maintenance planning and records people if an operator runs a mixed ETOPs/non-ETOPs fleet .... Alternatively, for example, you may need to run the APU to provide electrical redundancy during the ETOPs part of the flight .. and so on ...
(c) it follows that the operator must have appropriate systems in place to permit a reasonable probability of achieving the theoretical reliability figures tossed around during the ETOPs certification ..
(i) reliability monitoring and tracking is a requirement for the tech services people
(ii) the maintenance systems overall need to be reviewed and approved. Personnel need to be trained and approved ..
(iii) the pilot group needs to be trained and approved ...