turn my sims 4 package files into sims 3

Even though they are both .package doesn't mean that they are going to work. In fact, packages from sims 4 and packages from sims 3 are "completely" different to the point that they will not work on the other's. You have to specially create a new package using the sims 3 tools. There's plenty of tutorials out there, here's only for s4s. Try checking out the sims resource workshop for that.