Step 1: Understanding the Concept of Earliest Finish.
The earliest finish of an activity is the earliest time that the activity can be completed considering all its predecessors. It must be less than or equal to the earliest event of the following node because the activity cannot finish before the following event starts.
Step 2: Conclusion.
Thus, the correct answer is (3): the earliest finish is less than or equal to the earliest event of the following node.
Final Answer:
\[
\boxed{3. \, \text{Less than or equal to the earliest event of the following node.}}
\]