The other answers have their merits, but they miss a subtlety, I think.
The singular vs. plural depends on whether the companies / JV's are related or not.
Situation 1: the companies are "associated" with each other in some way, and they behave like one big(ger) company (e.g. alliances). In that case, it is correct to use singular "role", considering that one decision from the CEO will apply to all companies.
In this situation, the role is: "CEO of (X and Y and Z...)"
Situation 2: the companies are not related to each other, but the respective person is hired as CEO in all of them. In this case, he must make distinct decisions for each company / JV separately, he works with different people at different moments. In this case, it is more appropriate to use the plural, because the person is doing more jobs "in parallel", even if the jobs are very similar.
In this case we have several distinct roles: (CEO of X), (CEO of Y), (CEO of Z)...
Note: I am not sure if the situation 2 is legal or not in real life, but from a purely linguistic point of view, it is still a valid option.