Bună ziua! :) In short, we use "dintre" for plural forms, both male and female. And then, we use "dintr-o" when we are referring to a feminine noun and "dintr-un" when we are referring to a masculine one. I hope this was useful! :)

The words 'dintr-o' and 'dintr-un' are the combinations of the words 'dintre'+'o' and 'dintre'+'un'. The 'o' is undefined female article. The 'un' is undefined male article.

Thank you Venceslas and Adriana for your explanations!