Well, I guess it depends on the sales person but I was told that nothing changed until I got to 20 gB on a single device and then I could be throttled, if in a busy area. Also was told that it didn't matter if I was using the phone as a hotspot, which was my stated plan. I guess we'll see how it goes. The good thing is that the extra phone is on no contract, so if it doesn't work out I'll change.
I was also told that if one phone runs over it has no affect on the other two. As I said, "we'll see".
As for 2.4 or 5 MHz, I have no iea nor did i know I could control that. I laid each device on the corner of my bed and activated one after the other, checking speed on my laptop with speedtest.net.