I hold a twelve-month deposit plus a certificate for three years. How do I know what they earn together?

When you hold two products with different terms, the clearer way is to look at each on its own instead of adding them up. The deposit's return describes one year and the certificate's return describes three years, so their sum mixes two different periods and does not tell you what you will have on any day. For each product, look at the amount in it, the term, when the return reaches you (monthly, at the end, or upfront), and the total return over its own term. Note that comparing the deposit's rate directly with the certificate's is not accurate, because each term is priced on its own, and a fair comparison needs a common method across different terms. After the deposit matures, what happens to its money is not known now: if it renews automatically, its rate follows the product's terms at renewal, and if you withdraw it, your next decision will be at the rates available then. Ask the bank about the deposit's auto-renewal terms and how many business days you have to cancel it, and check them in the key-facts statement.