In a three-statement model, the income statement drives changes in retained earnings, the balance sheet tracks assets, liabilities, and equity, and the cash flow statement reconciles net income with the change in cash. Linking them ensures a forecast is internally consistent, for example that profit growth funded by slower collections shows up as lower cash.
Linked statements need a mapped opening balance sheet and operating schedules for items such as receivables, payables, inventory, fixed assets, and debt. When those inputs are incomplete, a good model says so rather than balancing with a plug figure.