Generally speaking, their are some rides where it really isn’t necessary to get a fastpass for. They tend to be less crowded so they have shorter or no wait times.
At Magic Kingdom these are: Mickey’s Philharmagic, Stitch’s Great Escape and Monsters Laugh Floor.
During the crazy busy times of the years these can have long lines. I suggest once you use your original 3 fast passes keep going back and see waht you can get and use. Noone likes to stand in line for long periods of time.