i used to barely be able to do one as well, but what I did was keep doing 1 pull up. I kept at it, and eventually I found it easier to do 1 pull up. So i challenged my self to do 2 pullups! Needless to say, it was epic phail for me, but i didnt give up, I kept at it, and in no time I could do 2. So I repeated the whole process to do 3 pull ups.
You will probably find that as you are able to do more pull ups adding on another gets easier and easier, for example doing 1 preullup then doing another is harder than being able to do 10 pullups and adding on another. Its all relative to how much you can do in the first place. Not exactly sure why, it could purely be a psychological thing.