If you're using a stovetop pressure cooker, three whistles generally means that the food is done. The time it takes to reach this point will vary depending on the recipe and the size of the pressure cooker. If you're using an electric pressure cooker, three whistles may not necessarily mean that the food is done. This is because electric pressure cookers can cook at different pressure levels, so you'll need to consult your recipe or manual to determine the correct cooking time.