Hi Taylor,
Thank you so much for visiting the Disney Parks Moms Panel with your question!
It sounds like you have a great grasp of the
Disney FastPass+ system. You are correct, in order to make a fourth Disney FastPass+ selection, you have to use your first three that you pre-selected. Then you can make a fourth, and once you have used that a fifth, so on and so forth for the rest of the day based on availability.
I understand your dilemma.
Seven Dwarfs Mine Train is a great attraction, and it does often build long lines. Having a Disney FastPass+ selection for this attraction is a great way to experience it with a shorter wait time! I suppose for me the answer to your question would depend on a few considerations. Consider for instance if you will be able to ride it on another day and how long you would be willing to wait in a queue for it.
Also, think about how early are your other Disney FastPass+ windows are. If they are done by 11:00 a.m., then I probably would cancel your 3:30 p.m. Disney FastPass+ selection for Seven Dwarfs Mine Train and make a different selection for 11:00 a.m. If they end at 2:00 p.m., I would probably keep the 3:30 p.m. Seven Dwarfs Mine Train selection. Another reason to keep this selection is if you plan on staying in the park late that night - then, if you ride Seven Dwarfs Mine Train at 3:30 p.m. with your Disney FastPass+ selection, you will still have plenty of time to make and experience more!
I know that I did not give you an exact yes or no answer, Taylor; however, I am hoping that thinking about some of the considerations above will help you to make your decision. Have a great time planning your magical vacation!