Depending on the CYA level, 3ppm may very well be too low and will not prevent an algae outbreak. Algae outbreaks are just the crack in the door for a pool store to sell you algaecides, Phosphate reducers, and other such chemicals that you either 1) don't need, 2) can get cheaper elsewhere (though can be convinced that being at the pool store outweighs the inconvenience of getting what you need on the way home.).