Will I have to do jail or placement

There isn't a way to really answer this question.  If you missed a court date three years ago and they are just now getting to you its likely you will do some or all of your time in placement.  The fact that you are on probation makes the chances of your freedom being taken away much more likely.  You should have an attorney looking into this.  There are ways of avoiding the worst penalties and most attorneys who practice in juvenile court can help you take steps to avoid the worst case scenario.  In generalist terms the answer to your question is it is very likely and they can hold you up until you are 21.