I doubt you would loose 3 mpg with a leveling kit. Now if you did a a full 3 inch lift maybe.?. I put a leveling kit on my 07 tundra and never saw a difference. A leveling kit is only lifting the front about two inches.
I would think a trucks drag is crated by its cross section already being "lifted" in the air (vs a car) lifting the nose an inch or two doesn't change the cross section area. It puts it a little higher in the air. Though like I said, it's already high enough in the air for air to flow freely under the truck.
I'm by no stretch of the imagination an expert on aerodynamics so my thinking may be way off base.
For whatever it's worth I plan on doing that to my truck one of these days