1 and 3 both work, depending on the conversation you're having.

Two of these could be used, depending on the situation. "I got to know her in 1998." This means that you learned a lot about her in 1998, and became friends at that time. Getting to know someone means that you become friends and learn a lot about each other. "I have known her since 1998." This means that you met her in 1998 and have known her ever since. This is probably the one you need.