First,
@Cogito it's not the cost. I offered and would be willing to to pay an additional fee. AndI also have the US and Canada.
Second,
@Kenny Phillips , No, I don't fly two airplanes at once. In fact, I don't fly at all. as I have been telling people on this site and others for a decade, I lost my medical 45 years ago. But my wife is IFR rated. She does all the flying and she keeps her mini-5 in a holder on the center arm of the yoke. I keep my Pro in my lap. I use it to look up all sorts of information during flight so she doesn't have to. I am also a second pair of eyes to look for traffic on ADSB-IN on my Ipad and to look for airspace restrictions. I look up frequencies, runway directions, and a lot of other things. As for the Iphones, we would both like to have FF on the ground for checking METARS and TAFs, planning various trip scenarios and looking up a lot of other information that is admittedly available elsewhere, but we like to use Foreflight. I don't think we have ever been on vacation when we didn't use one of our devices to find out where the nearest airport is and how far away other airports are.
The three device limit is not a deal breaker and it is not a heartbreaking hardship. But if we could use 4 devices, it would be more fun and more valuable to us. And like I said, I wouldn't minding paying more. But not 100% more for a 4th device.