A possible theme could be the search for the true meaning of marriage. In the first two marriages, the character may have had wrong ideas about what marriage should be. But in the third marriage, they finally understand that it's about sharing a life with someone in a way that is based on love, equality, and mutual support. It could also be about learning from past experiences and using that knowledge to build a better relationship in the third marriage.
One possible theme could be redemption. After two failed marriages, the character may find redemption in the third marriage, making up for past mistakes. Another theme could be growth. Through the experiences of three marriages, the character grows and changes.