I think any day would be fine, honestly. But just in case all three days are equally bad I would say to go to the lower lot immediately as possible since rides like The Mummy, Transformers, and Jurassic Park are usually the most popular and have really long wait lines as soon as the park starts to fill up. A Studio Tour is also a must and you should go on that first, followed by the lower lot. I've done the studio tour first, the lower lot, then the upper lot last since the upper lot just consists of simulator rides.