马上高三了，用英语可以说：

1. We are about to enter Senior Three.

2. We are going to be in Senior Three.

3. We will upgrade to senior three.

4. We will be in senior high 3.

5. We're to be in grade twelve soon.

这些表达都表示即将进入高三的意思，可以根据具体语境选择合适的表达方式。