Your first three Fastpasses have to be at the same park, even if you have the park hopper. After you use up your initial three at the same park, you can book one additional fastpass at a time at any park, for any FP that's available. Keep in mind that you have to scan in for these three FPs before you can book the next at any park, so these additional can't really be looked at and decided until after you scan your first FP. A nice little trick for the additional FP's is that it's not based on actually riding the ride nor the hour. For example: You can scan your final FP at (let's say) Haunted Mansion. The minute you scan your FP, you can look for another one while you wait in line to actually ride the ride. Technically, you don't even need to ride the ride if you don't want to. I've done this before where we were trying to use up our last FP of the day because we wanted to switch parks, so I booked a FP for Bugs Life, scanned our FP's and then walked away, and was able to book a FP at any park.
Hopefully that all makes sense!