Write the number in expanded form as a decimal number.

To write the expression \( 3 \times 100 + 4 \times 10 + 1 \times 1 + 2 \times \frac{1}{10} + 9 \times \frac{1}{100} + 3 \times \frac{1}{1,000} \) in decimal form, let's calculate each term step by step:

1. \( 3 \times 100 = 300 \)
2. \( 4 \times 10 = 40 \)
3. \( 1 \times 1 = 1 \)
4. \( 2 \times \frac{1}{10} = 0.2 \)
5. \( 9 \times \frac{1}{100} = 0.09 \)
6. \( 3 \times \frac{1}{1,000} = 0.003 \)

Now, we will add all these values together:

\[ 300 + 40 + 1 + 0.2 + 0.09 + 0.003 \]

Let's add them in steps:

• \( 300 + 40 = 340 \)
• \( 340 + 1 = 341 \)
• \( 341 + 0.2 = 341.2 \)
• \( 341.2 + 0.09 = 341.29 \)
• \( 341.29 + 0.003 = 341.293 \)

Thus, the expanded form as a decimal number is:

\[ \boxed{341.293} \]