My question is this, my youngest child has tuberous sclerosis, how come my other two do not?

TS is a condition caused by a dominant gene mutation. In some patients it is a brand new event in their genes. In some, the inherit it from a parent who may have a very mild case & not know they have it. If a parent carries the defective gene, the risk is 50% at conception to get it. The intensity of expression is variable. Please consult with a clinical geneticist for your family.