The best thing to do if your best friends have ignored you for three days is to try and talk to them. You need to try this so that you can get to know what is going on as this is the best option. If you try talking to them and they still continue to ignore you, you should also do the same since you did your part as a friend.
ignore them back. if they are going to be childish, then be the adult and get over it. when they finally come around, and they will!! just tell them your busy and walk off. find new friends. dont associate yourself with petty people
Obviously they are punishing you for something you've done. Keep meeting them for lunch and when they think you're sufficiently "punished" or when they get tired of punishing you they'll probably tell you what you did wrong. When they do, just say "sorry" and it should blow over.
It helps reduce the "punishment" if you bring dessert :)