The answer may depend on what your car's electronics are doing when the car is parked - for example things like immobilisers. Three short journeys a week may not be enough to compensate for starting the engine, especially in winter with a demand for headlamps and blowers etc. If the battery gradually gets drained under this regime it will show its age sooner, so give it a trickle charge every month or so. Also make sure the terminals are cleanly connected.
I have a 1991 Pug which goes on an 80-mile run every 2 or 3 weeks, and the battery always seems to need very little topping up, but if it has not been out for several weeks I give it a couple of hours trickle, just to make sure.
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