Each person is only guaranteed three Fastpass+ credits for each park, so you'll want to get the most out of them. Do some research into what rides and attractions are can't-misses for your group, prioritising Fastpass+ for those with the longest queues. Then, try to book them as early as you can (30 days in advance, or 60 days for those staying at a resort hotel).
Bonus tip: Be sure to read the fine print, as some of the parks (such as Epcot) have tiered systems that restrict the use of more than one Fastpass+ in a top tier (you will have to choose between Frozen Ever After and Soarin'), whereas others (like Animal Kingdom) may make you choose between certain high-demand rides (such as Avatar Flight of Passage and Na'vi River Journey).