It can start at any time, and 3 years old would not be an unusual time. Rather than E45 cream, go to the pharmacist and ask for a more gentle specialist children's eczema cream. It should be rather more gentle.
My dd2 got eczema this summer for the first time, and she is nearly 6. Her trigger seemed to be sun screen. I have switched her to an organic brand and the eczema is going. Occassionally if eczema is late onset it is possible that there is a simple trigger. Unfortunately this is not the case for many people.
E45 also made her very unhappy. Aqueous cream was much gentler.