1. **State the problem:** Find the first three terms of the sequence defined by the formula $a_n = 20 - 3n$. 2. **Formula:** The general term of the sequence is given by $a_n = 20 - 3n$, where $n$ is the term number. 3. **Calculate the first term ($n=1$):** $$a_1 = 20 - 3 \times 1 = 20 - 3 = 17$$ 4. **Calculate the second term ($n=2$):** $$a_2 = 20 - 3 \times 2 = 20 - 6 = 14$$ 5. **Calculate the third term ($n=3$):** $$a_3 = 20 - 3 \times 3 = 20 - 9 = 11$$ 6. **Final answer:** The first three terms of the sequence are $17$, $14$, and $11$.