Final answer:
Create a specific style for subheadings in the book using Times New Roman, 14 point, bold, and centered format.
Step-by-step explanation:
To ensure all the headings in a three-hundred-page book have the same format, the designer should create a style specifically for the subheadings. The subheadings should be Times New Roman, 14 point, bold, and centered. This can be achieved by creating a paragraph style or a heading style in a word processing software like Microsoft Word. The designer can then apply this style to all the subheadings in the book to maintain consistency.