On the back of his signing as Williams' test and reserve driver, the team's technical chief, Paddy Lowe, underlined that Robert Kubica was particularly signed to make use of his experience and technical knowledge to help develop the FW41.
This forum contains threads to discuss teams themselves. Anything not technical about the cars, including restructuring, performances etc belongs here.
Don't see why Ocon can't be reserve for all three, the teams are closely linked and requiring the services of a reserve driver is rare, the odds of more than one team needing him at the same time are very low.
Ocon has already tested for Mercedes several times while being a Force India race driver, even if he doesn't have a seat Mercedes will be keen to give him every chance to drive possible so in-season testing for both FI and Mercedes is a certainty. Russell too will probably get inseason testing for Mercedes as well as Williams, he's already driven the Mercedes this year.
Also, Mercedes probably does not see FI and Williams as serious rivals for wins or Constructor's Championship so Merc is comfortable sharing their good sim/reserve driver if it adds value to their customer engine package. In contrast, Mercedes would not be willing to share Ocon with Ferrari or Red Bull (ha, Ocon-RB would be an interesting marriage right now!).