Given three days and on a first time visit, I would recommend just sightseeing, too.
Yet, if you really like your beach break, Barcelona has the huge advantage of being a large city with the beach right there. You might include the beach like many locals do, after work resp. in your case after sightseeing, for two or three hours. It is just a bus stop away, or from some sight a walk of less than 15 minutes.