Unfortunately, options are very limited in a 3g tank due to bioload. I'm not sure I would be comfortable adding anything else to it, if it were me.
The best way to proceed when being overcome with algae is to first identify exactly which kind you are dealing with, as each tend to appear with a certain imbalance of nutrients and/or light, or are normal with "younger" tanks (diatoms for example). http://www.plantgeek.net/article_viewer.php?id=9
Once you know which type you have, you can correct whatever imbalance is causing it. I'm not a big fan of getting a species specifically for algae cleanup unless you are interested in it before hand; and are going to be happy with it, and it will fit into your community when the algae problem is under control.
Note, not all algae eaters will consume all types of algae. There are many that each will ignore entirely. For example: diatoms, the oh so pretty brown slime, there are really only a few species I know of that consistently eat it: otos and nerite snails. Brush algae is a giant pain; but is devoured by Siamese algae eaters (Crossocheilus siamensis). So dietary preferences are other things to consider, along with your tank size and species compatibility.
So step one in this situation, is to figure out which type of algae you have. :) Once you know that, you can decide how best to handle it.