I've met people who are polyamorous, which is what it sounds like you're referring to. Basically you have a primary partner (girl a) and then you have a secondary or tertiary (girl b, girl c) and you all date casually or seriously. Some people have a wife and girlfriend, some people will seek out lesser secondary relationships. It depends on what you and your primary are comfortable with and what you can handle. Obviously, it takes some effort to make sure that everyone is comfortable and feels that they are getting what they need from the arrangement. Some poly couples will form a "triad" where they both date the same third person, which is what your dream sounds like.

Alternatively, you can find a lot of secondary partners that aren't interested in a romantic setup but a sexual one. Basically you'd have a primary romantic/sexual partner and then a secondary sexual partner. There's no limit to who and how you can date, so long as you make sure everyone is getting their needs met.