If you're tired, take a break. Eat something, get some rest or take a walk outside. If you're out of inspiration, you may want to watch a movie, read some book or listen to some music. In your essay, you may want to plan out its structure and think which parts could use some extra details. Write all you know about something, give some examples to make your essay more concrete, or try anything else that works. Usually it's not that hard to write three pages worth of text. Just make sure you stay on topic in your essay.