Losing my mind (well, one-third of it)

I have a lot of ideas, and when I want to express them I create verbal lists, even in everyday conversation. I often say things like, "I have a few thoughts on this. One, I think blah blah.... Two.... Three...." On the way to lunch with a new colleague, I said, "I only know three restaurants in this area: an Indian restaurant, Jupiter, and ..." and it was gone. "... I guess I only know two restaurants in this area." My new colleague cracked up, thinking me a comedian. And it was funny, until later in our lunch I tried to run through another list of three and could only come up with two, which I covered more subtly. And it happened again later.

Sunday I went to the store to get three things. On the way there, I recited them to myself. Except I could only think of two. Thought hard, thought hard ... and remembered the third. And forgot one of the other two. It took me most of the way to the store to get all three to stay in one place in my mind. It was like herding mental cats. Or mentally herding cats.

Where did that third go? Did I learn something that took the brain storage that I needed to remember the third item in a list?