I started with posting the first three chapters separately and I found that I got a fair amount of traction at the beginning (about 40 subs per update, each update a month apart). I don't have another instance to compare it too and technically it's possible that I would have gotten even more traction had I waited to release the first three chapters together, so take my personal experience with a grain of salt.
Of course there's other variables to take into account as well. Does the first chapter as a standalone do a good enough job at interesting the reader? Is there even enough content in the first chapter to release it by itself? Can you afford to release three chapters all at once? There isn't really a hard yes or no answer to your question, so probably just do what you're most comfortable with.