Never heard of the 3 months thing unless you are talking about non aquatic plants like some of the ones that petsmart sells. Many factors determine how well plants will do in your aquariums, including the type of lighting you have. A very easy plant I like is hornwort (Ceratophyllum demersum)as it will grow in almost any tank. Some fish will eat it though so if you have large fish that like plants it may not last long.