"第三部分" can be translated as "Part Three" or "The Third Part" in English, depending on the context and the style of writing.
"Part Three" is more commonly used, especially in lists, headings, or when referring to a specific section within a larger document or presentation.
"The Third Part" can also be used, but it might sound slightly more formal or literary in some contexts.
In most cases, "Part Three" would be the preferred choice.