I have a diamond ring with a total diamond carat weight of 1.5 carats in small diamonds. Are they worth the same as a single 1.5 carat diamond ?

No, the value is not even close. A single 1.5 carat diamond with the same shape, cut, clarity and colour will be worth many times more then the 1.5 carats of small diamonds. This is mainly because large diamonds are rarer, and much more valuable.