If different UY then you need to drop down and select which on the booking page (the page where you select what dates and room type and resort). It shows the one membership/UY and you click it to drop down and select other.
If one UY/membership then when you book it will allow you to select which contracts to book with. It will allow you to select use 1st, use 2nd and use 3rd. It will be after you select what you want to book. But will use banked and borrowed first in all contracts before going to current points. And if in 7 month window it will use those from each of the 3. Say you have 3 contracts and 2 have banked points, one has 20 banked and other has 30 banked. Say you book a resie for 65 points, it will take the 50 banked and then go to the one you slated to use 1st to take regular points.