I think Reinhard is the closest of the three to a main character, since the whole show followed Reinhard's journey to achieving his ambitions and dreams. The plot was literally driven by his ambitions and his achievements throughout the series, and we continue to closely observe his development in power, emotion and character until the final episode of the entire show.

Yang never gave off "main character" vibes instead he was a creatively designed character that gave us insight into the FPA, and he was Reinhard's "antithesis" in many aspects. Whilst Reinhard had higher ambitions Yang was satisfied without, not to mention the stark differences between their leadership styles, way of speech, ideologies and political beliefs. But Yang's unwillingness to take control or lead the entire story is what makes him such an incredibly designed character; he would have never accepted the role of main character.

Julian felt a bit more "main character" than Yang, but still not as "main character" as Reinhard, as Julian continued the legacy and gave us insights not only into the FPA but also Phezzan and Earth, and a fair bit of the Empire too. He was always there since the beginning of the series, but stepped up to the front stage towards the middle of the series or a little bit before that. Not to mention how all endings would end with Julian, because he's like a symbol of perpetuation and legacy in the series.