Step 1: Understanding the Concept:
The Preamble to the Indian Constitution has been amended only once since its adoption.
Step 2: Detailed Explanation:
The 42nd Amendment Act, 1976, added three new words:
1. Socialist
2. Secular
3. Integrity (It changed "unity of the Nation" to "unity and integrity of the Nation").
"Unity" was already there, and "Federal" is not in the Preamble.
Step 3: Final Answer:
The three words added were Socialist, Secular, and Integrity (A, B, and C).