You begin with the three set of One blue,one brown,one yellow,for left,right and center.
It's an easy level for me,maybe because of medals.
You should get Decamaster medal,make level-10 tower of every kind.
Just cleared level6 and withstand 23minutes in survival.
It's possible.
You can earn medals in level3 by not building landmark and makign lots of towers.
 "Refraction"){kind=icon}
 "Silver Maze"){kind=icon}
 "Zilch"){kind=icon}
 "Besieged"){kind=icon}