(3x² + 5xy + 7x²y) - (5xy + 3x²) - (7x²y - 3x²)

1. Distribute the negative signs: 3x² + 5xy + 7x²y - 5xy - 3x² - 7x²y + 3x².
2. Combine like terms: (3x² - 3x² + 3x²) + (5xy - 5xy) + (7x²y - 7x²y) = 3x².