Currently with FastPass+ each guest may book 3 per day. During the testing phase Key to the World cards and hard tickets of off-site guests still work in the regular fastpass machines. So, technically you can have your 3 FastPass+, and however many regular fastpasses you care to have. This will change once the system is at 100% implementation (we don’t know when that will be). I don’t know why the current number of FastPass+ is 3–Disney probably made that decision based on ride capacity and other research.