五下英语三单元作文的主题是“我的朋友”，通过描述朋友的外貌、性格和日常活动，来表达对朋友的喜爱之情。以下是作文示例：

My Friend

My best friend is Lily. She has long, curly hair and big, bright eyes. She is very friendly and always ready to help others. Lily is also very kind-hearted. She often shares her snacks with me during lunchtime and lends me her books when I need them.

Lily loves playing sports. Every afternoon after school, she goes to the playground to play basketball with her friends. She is very good at it and can score many points in a game. Besides sports, Lily also enjoys reading books. She reads a lot of stories and always has new ideas to share with me.

Last year, Lily helped me a lot when I was feeling sad because of my grandmother's illness. She listened to me carefully and gave me some good advice. Thanks to her support, I felt better soon.

I am lucky to have such a wonderful friend like Lily in my life. I love her very much and hope we can be friends forever.

This is my friend Lily, a kind-hearted girl who loves sports and reading. I am proud to call her my best friend.