Do you want the entire house cleaned more than once over the three days, or do you want all rooms cleaned one time? (Ex: Living areas, kitchen and baths M, bedrooms W, kitchen and baths F vs. bathrooms and kitchen M, living areas W, bedrooms F)
And how thorough do you want the cleaning to be? Do you want the full bath done in 15 minutes or in 30?
Do you plan to have the menus planned and needed items on hand or will you want her to shop as well each day or weekly?
Depending on your answers, I can see this being a 15 - 20+ hour a week job, with a higher rate than a FT housekeeper would get because of it being PT, so maybe $20+ per hour, and PTO based on her hourly work time would be a great bonus.
You might also see if any of your neighbors or friends need housekeeping help, because offering FT work, even if it's with 2 - 3 families, would seem to be more appealing than PT, since she would then need to find other work on her own that fits outside of your needs.