I found web sites for two of the three. They are:
Delta Gamma and
Kappa Alpha Theta. Kappa Kappa Gamma is the newest at Harvard, so they may not have a web site up yet.
Also, here some articles on sorority life at Harvard:
http://www.thecrimson.com/article.aspx?ref=269508
http://www.thecrimson.com/article.aspx?ref=347587
http://www.thecrimson.com/article.aspx?ref=349063
I strongly recomend joining a sorority, especially at Harvard. This university has a reputation for being one of the finest in the world, but that reputation does not extend to student life. This is why sororities have recently appeared in Cambridge. They are filling a need. A sorority will also help you to quickly get involved in other campus activities. Most importantly, you will have a new family to help your stay at Harvard be more fulfilling.
Best of luck!