well if your FP Production was predominantly 3s, you'd gain way more from having 1 at a time up for 3 times as long than 3 up at once.
in practice I suspect most cities will accidentally average out to 20%-ish with any number up at once. here's my beta city:
View attachment 9417
(colors: red = worse than 15% for that number of temples for that size of FP producer, orange = 15-19%, yellow = 19-21%, green = 21-25%, dark green = > 25% per building)
1 at a time is ever so slightly ahead for me, and 3 at a time would actually be the worst selection.
The ".6" in buildings that make 10 was a 40% chance of 40 FP from pirate hideout. 0.4 40s or 1.6 10s come out the same because neither rounds off when you proc it and it saved me from adding in a whole bunch more lines.
In general, unless your FP production is predominantly from one building value I don't think you have to worry about how it rounds too much.