If you get an App that is normally 3$'s but for a special it's Free for one Day do you have to pay afterwards?

It's free for good, don't worry. They can't just decide to make you pay for it, that would be robbery. If you go to the supermarket & get bananas on sale, & next day they're full price again, there is no way they can make you pay for it! Don't worry, you got a great deal. I always get apps free and then come back to it later and its a crazy price. You're safe!