You can pre-book up to 3 fastpasses per day. Those need to be in the same park.
If you pre-book 3 fastpasses, you can make more one at a time as soon as you tap in for the 3rd FP. These additional ones can be at any park with no restrictions.
For park hopping only:
If you pre-book only 2 fastpasses, you can book a 3rd in any other park immediately after tapping in to the first one. If you decide to stay in the same park, you can still book so but you are restricted by the tiers. After using FP #3 you can then book more, one at a time.
If you pre-book only 1 fastpass, you can book two more in any other park(s) immediately tapping in to the first one. You are then restricted by the tier rules in the second park, so only one may be tier 1. However you can book a tier 1 at one park and another tier 1 at another park (so you would be visiting three parks in one day). After using FP #3 you can then book more, one at a time.