Which one of the following sentences is more correct?

"Maria's, Mary's and Sophia's first loves was Katarina.

"Katarina was Maria's, Mary's and Sophia's first loves​"

"The first love of Maria's, Mary's and Sophia's was Katarina."

I'd go for the second statement in your list but with a correction - "Katarina was Maria's, Mary's and Sophia's first love."

It looks simpler than the rest.

The first sentence also looks fine, but again with the small correction - "Maria's, Mary's and Sophia's first love was Katarina.

The last sentence would be my last preference.

Although, I should add that the order of preference will vary depending on what you're trying to emphasize.

Hope this helps.

NS