The 3 FastPasses that
DCL will give you can be used on any ride, any park, any time. Of course they couldn't put restrictions on them and not allow you to use them on the new TSL rides but last year during these cruises you could use them on the Pandora rides so hopefully it'll be the same this year. We're on the Nov. 10th sailing and planning on using one on Slinky Dog, and then the other 2 at MK.
According to people who took this cruise last year, you can link the ticket they give you to your
MDE account and actually make three FP+ reservations before you get to the parks that morning in addition to your 3 anytime Fastpasses. Once you use those three you can continue to make additional reservations based off availability. It sounds like last year everyone got their ticket cards the night before so most people linked it to MDE once of their bus to the parks. Of course there won't be great availability but it can still help save time.
Long story short: You'll get 3 anytime Fastpasses, and if you're familiar enough with MDE you can also link your ticket and make 3 reservations that follow standard FP+ procedures. This means you can start your day with 6 Fastpasses.