If I was going to spend all day in a single park and didn't have to plan around lunch or a midday break I'd most likely schedule FP+ between 11:00 and 3:00. Strategy would be to take advantage of shorter standby lines for a couple hours after rope drop, use the 3 FP+ when the hoards begin arriving, then have the late afternoon and evening to get more FP+.
Of course it isn't that simple for us, because we park hop a lot, have meals to plan around, and take midday breaks.
Our primary strategy is to hit a park at rope drop, take advantage of shorter lines early on, take a midday break, then hit a different park in the afternoon/evening where we have FP+ scheduled. Without FP+ for the second park you wouldn't be able to accomplish much of anything.
That strategy doesn't result in the maximum number of FP+ used in a day, but it does save the most time for the 3 you use, because lines are at their worst in the afternoon/evening.