Step 1: Understanding the Concept:
The Preamble has been amended only once since its inception, through the 42nd Constitutional Amendment Act of 1976 during the Emergency.
Step 2: Detailed Explanation:
The 42nd Amendment added three specific words:
1. Socialist: To formalize the state's goal of social and economic equality.
2. Secular: To emphasize that the state has no religion and treats all religions equally.
3. Integrity: It changed "Unity of the Nation" to "Unity and Integrity of the Nation".
The word "Unity" was already present, and "Federal" is not explicitly mentioned in the Preamble.
Step 3: Final Answer:
The three added words are Socialist, Secular, and Integrity (A, B, and C).