How much is a 3 carat marquis diamond worth?

Any diamond is worth whatever someone will pay you for it.

Today, on Blue Nile, for example, you can purchase such a diamond and spend between US$25,757 and US$187,712 , depending on the clarity, colour and Excellence of the marquis cut weighing about three carats.

Best practices dictate that if you own such a diamond that you pay a certified gemologist to document the diamond, so that you have a better idea of how much to ask someone to pay you for it.