Well, in my experience, lesbian and bisexual can both be used as both nouns or adjectives. Gay is the only term that is usually only used as an adjective. It may have some to do with the fact that "gay woman" is a term that can be used in place of lesbian, so gay as a noun isn't descriptive on its own to refer to men specifically, however I think there's another reason. When christian religious homophobes prootest against our existence and rights, they tend to say, "the gays" to refer to us. While they may mean LGB in general, they're probably sexist and bi-ersasure-ish enough to not really care about LGB other than gay men. As such, the term as a noun has gained a very stained connotation and so it is now only used as an adjective