Dom, I've kind of use all three methods (one will be used for the first time this year). I think of your three designs, I would go with #3 as well.
Last year I incubated corn eggs with a Styrofoam cooler, water heater and an egg box with vermiculite. My temps were pretty steady but I only got about a 75% hatch rate. Not sure why...
I also used (for more corn eggs) a fridge with heat tape all up the back and, for the most part, kept the humidity in the egg boxes. I did have some conaitners with open tops filled with water on the door of the fridge. Again, my hatch rate wasn't the greatest... probably about the same as my first clutch/method.
This year, I built (for python eggs) a big wooden box, lined it with Styrofoam and poly (plastic wrap) and have about 80L of water in a LARGE bin being warmed by a 200W heater. My trial boxes have maintained 89F and high humidity. They have wet paper towel in the bottom and the eggs will sit on egg crate with glass on top for viewing.
With my fridge this year, I am trying less ventilation holes, no substrate and no humidity in the fridge whatsoever. We'll see how that goes. So far I've found the humidity is getting sucked out of the boxes. I'll try new boxes with less ventilation though.
Anyway, just thought I'd pass on my experiences with the different types I've built.
Good luck!