If your enclosure holds heat well, that hot spot isn't going to be all the heat they get. Obviously that 3 inches will have a higher surface temperature than the floor around it, but it's also going to raise the ambient temperature above it. Like Connie was saying, our snake room is heated to 83 degrees all year long. With that 3" hot spot, it raises the ambient in the back of the tub to around 90 degrees or so.