You could get a superlight racing bike, but by the time you add your fenders, lights, a lock or two, rack and panniers, etc, you will be hauling over 30 lbs up and down the stairs. Most mid-level hybrids are probably close to 10 lbs heavier than light racing bikes. Calculating the weight you may need to carry is left as an exercise for the reader.

'Commuter' bikes are, AFAIK, hybrid style bikes that come equipped with the fenders and racks, etc. that most people buy separately.

For a 3 mile commute, however, just about any bike will work.