A semicolon (;) is used to combine two separate sentences with a similar topic. The two sentences that are trying to be combined are "My Uncle Bill has such a beautiful voice" and "many couples has asked him to sing at their wedding". Since these two sentences share a similar topic, a semicolon would be best.
The correct answer is the third one. The one that says "My Uncle Bill has such a beautiful voice; many couples have asked him to sing at the weddings." This is an instance when you would use a semi colon instead of a comma, most people would think to use a comma but not in this case.
