If I was subpoenaed as a witness to an assault but it has been continued 3 times, does my employer have to pay me for the missed days of work?

No, unfortunately, your employer does not need to pay you for the days you missed from work to serve as a witness. The employer is not required to effectively subsidize the legal action in that fashion, by paying for witness time, and is also not required to pay employees when they do not work.