My Dierama is 3 years old. Can I divide it?
You shouldn't have to divide this plant for at least five years unless it's already become overcrowded or flowering is reduced. If that is the case, clumps can be divided in spring.
Best season to divide? Does the foliage need to be cut back before digging? How many corms should one replant in order to have a strong stand ?
It is less about the season that you can divide, and more about letting them multiply until their flowers start to slow. This will happen after about 5 years. Here is an article that will explain better: https://www.gardeningknowhow.com/ornamental/bulbs/wandflower/growing-dierama-wandflowers.htm