Go into Avatar Famestar > Famestar Challeneges to check the weekly challenges. For the achievement you only need to complete 3, there are 10 different ones each week and most of them are pretty stright forward but a bit time consuming as they are "weekly" challeneges. Some of these require you to go online and perform certain tasks so you will be switching modes to get the all the ideal challenges you want to get done. The Bonus challenges don't count as weekly ones just to make sure.