I'm aiming to ask people around February. I'll start working on my PS this break and January, get it proofread and all that jazz, and then hand in a packet to any people I'd want to write me letters. They're all people I did very well in their classes and got to know, but they might not know my motivations, so I just want to be prepared to present them with a resume and PS and offer to "interview" with them about why I want to be a doctor, just so they have an idea of why I chose this path.
It's not too early, but you might end up meeting a lot of people in your 3rd year that you ultimately end up getting to know much better. There were people that I considered asking in my 2nd year that I ended up not asking at all by the time I was actually applying. It never hurts to keep the letter on file, though.