Yes, the F-150 running boards just come with two brackets. I've not bolted the middle bracket to the truck yet and I've not noticed any issues with flex or anything. Just was surprised that the lugs were there for the middle brackets, and since they are, I do plan to attach them.
Figured my first post would be here. I'm old and my wife has mobility issues so I installed F250 OEM running boards. Don't hate the look and function is much better. Was pleasantly surprised to find that the F150 had the studs for the middle set of hangers.