Three is first because it is a cross-cutting relationship.
One is next because of superposition.
Then six because its below 1 and in superposition whatever is on top is the youngest.
After that is two because it is a cross-cutting relationship and sense it's that its younger than all the surrounding layers.
Then it's five because of superposition.
Finally it's four because it's the lowest layer that's not a cross-cutting relationship layer.