Not a native speaker, but I've been studying Spanish for 5 years.
Eso refers to a general concept or idea most of the time. Kind of like 'lo' when you say, "Quiero ir al cine, pero no puedo hacerlo." I want to go to the movies, but I can't do it. (Not the greatest example, but the concept is the same.) Eso, however, does not mean it, rather it means 'that.' "That won't work." "That general idea won't work." It's used for stuff that isn't gendered. Also, I've seen it used like, "Qué es eso?" What is that? Maybe it's something that has a gender in Spanish, but you just don't know what it is, so you use eso.
Ese and esa mean 'that' as well, but they have to agree with the noun. Ese gato... Esa camiseta... It's as simple as that. (It's as simple as 'eso'; hahaha, I know I'm not funny.)
Hope that helps, and, like always, native speakers, go ahead and correct me if I misspoke!