I don't usually think about what my kids write in terms of sentences, but in terms of details they've included and the value of those details. Here's what I would like to see from a first grader at this point in the year:
On Saturday, I was at Tim's birthday party. There was a ball pit filled with red, blue, and yellow balls.
"Whoppeeee!" I yelled. I took a big running start, tucked my knees in, and made a cannon ball into the balls. They exploded everywhere. Some other kids screamed and laughed.
I pretended that the balls were water and I was a swimmer. I could lay on my back and not sink! I pretended that the balls were snow and I was plowing it with my hands. When my mom came to the party, she had to drag me out of that ball pit!
Of course you aren't going to get that much from every kid, but if I were modeling it, that's what I would write. What I found the first time I did the units, was that I really had to hone my own writing. I had to get out of the "list" mindset that I used to model. I had to literally stop myself, sit in the mornings, and think hard about how to write my own small moments. A lot of people tell me that they struggle with getting their kids to write small moments, but I think that the key is in daily modeling (mostly oral).
My opinion is not to let them move to listing three things. They are able to produce better than that. When you get to later lessons, ESPECIALLY in my favorite unit (Revision) you'll wish you didn't do that. Later units are going to offer strategy after strategy for stretching these moments. Stick with it! You'll be AMAZED!