for creating a triangle we need 3 vertices we have n vertices choose one in nc1 ways .as the probability of edge on that vertex is 1/2, so 1/2* nc1.
again from remaining n-1 vertices choose one with probability 1/2 i.e 1/2*n-1c1 .again from n-2 choose one i.e 1/2*1/2*1/2*n*(n-1)*(n-2) now divide it with 3! as it is unordered graph.
i.e 1/2*1/2*1/2*n*(n-1)*(n-2)/3! put n=8 you get your answer.