Generate a full, in-depth guide on how to build a 3-year financial projection model in a spreadsheet. The guide must be a practical, step-by-step tutorial. It should include detailed sections on: 1. Assumptions: A guide to documenting your key business assumptions. 2. Sales Forecast: How to build a top-down and bottom-up sales forecast. 3. Expense Budgeting: A detailed breakdown of fixed vs. variable costs and how to project them. 4. A template and explanation for creating a projected Income Statement, Cash Flow Statement, and Balance Sheet. Please include a strong disclaimer to consult with a financial professional.