'It's important to say that it's impossible to make specific comments without meeting a patient in person. Similarly, without the full staging information as well as the subtype, grade and other clinical features it is hard to say as these are essentially used to make a definite treatment plan.
With that in mind, the international standard of care for women with stage III ovarian cancer, in particular if they have high grade serous disease (the commonest subtype), is carboplatin and paclitaxel. However, if you look at the clinical trials that were performed in the 1990s, none of them showed convincingly that carboplatin and paclitaxel was definitely better than carboplatin alone. Nonetheless, the combination has become completely standard.
There may be a group of patients who definitely need to have both drugs, but we still don't have definite information on who those patients are. There are definitely circumstances where single agent carboplatin is appropriate - firstly, in women who have pre-existing nerve damage, for example from diabetes. Secondly, for women who do not want to lose their hair.
Finally, patients who are frail can find combination chemotherapy very tough. I’d therefore suggest you and your mother-in-law write a list of all the questions, and make an appointment to see the oncologist again to discuss all of the above. It would be helpful for her to have someone with her at the appointment to make notes, as there’s always a lot of information to take in.