Learn Farsi: هر کارشناس در خصوص این موضوع مهم نظر خود را می‌گوید. - Every expert gives their opinion regarding this important topic.

Yes, they all mean "about" or "regarding" and can be used interchangeably in most contexts. However, dar khosus-e sounds a bit more formal and precise (like "concerning" or "in regard to"), which fits perfectly in a sentence talking about experts and an important topic. râje' be is the most conversational of the three. Remember that dar khosus-e and dar moured-e need the ezafe (the hidden -e sound) to connect to the next word, while râje' be does not.

The English translation says "their opinion". Why does the Farsi use khod instead of a possessive ending like nazaresh?

khod means "self" or "own" and is used to refer back to the subject of the sentence (har kârshenâs). While you could technically say nazaresh (his/her opinion), using khod is very common and natural in written or slightly formal Farsi to clearly mean "their own opinion". It emphasizes that the opinion belongs to the expert speaking.

Why is kârshenâs (expert) singular when we are talking about a group of people giving opinions?

In Farsi, the word har (every/each) is always followed by a singular noun. So just like in English where we say "every expert" rather than "every experts", Farsi uses har kârshenâs. Because the subject is singular, the verb at the end of the sentence, miguyad (says), is also singular.