As Douglas says, you're meant to check the oil weekly, topping up when necessary. (You're also meant to check other things, such as your tyre pressures, at the same time).
I admit to not being perfect though. I probably check the oil level for my elderly, high-mileage Saab once every few weeks, plus before starting out on any long journeys.
I wouldn't regard using a few litres of oil every three months as being particularly unusual with most cars, particularly with a fairly high mileage on them.