Fastpass+ Question

I understand you don't have to book 3 initial fastpasses anymore; but in order to get a fastpass in another park do you have to book all 3 in the first park then 4+ are hopper passes only? Ex: can I book FEA then hop to MK for 7Dwarfs & BTMR? Or do I have to book three at Epcot before I can try to get fast passes at the magic kingdom?