Can adjacency lists be used in directed graphs?
Your graph is actually undirected. The notation is a list of edges in the graph. Since edges are undirected, each edge is a set of two vertices. Your lists contains three edges, since a triangle has three edges. It actually contains all possible edges involving the vertices $a,b,c$.

A directed graph is one in which edges consist of two ordered vertices. The edge points from the first vertex to the second. You can represent such a graph using a list of pairs.

Adjacency lists are a different data structure, which is described in the Wikipedia page you link to, so I skip its description here. It can be used for both undirected and directed graphs, in the natural way.