在读高三的英语表达有多种方式，以下是一些常见的表达：

I am in my senior year of high school.

I am a senior high school student.

I am currently reading English in my final year of high school.

I am in my twelfth grade.

I am in Senior Year.

这些表达方式都可以准确地传达出“在读高三”的意思，可以根据具体语境选择合适的表达。