No fast passes during hard ticketed events (MNSSHP, MVMCP, etc). If you schedule your first 3 fps early in the day, there's plenty of time to continue to add fps up till about 6:00.
If you weren't going to a park that day just the mnsshpMDE would allow you to use that ticket to make FP to use during the 4-7 window (3:30-6:30) but MDE will not allow you to make 2 sets of FP with 2 different tickets for the same person for the same day. You can use the 3 original ones in the am and continue to schedule additional once those are used (however late afternoon availability will be more scare due to MNSSHP tickets being used to book FP for people without regular tickets for the day)