Sylvia - maybe I don't understand your question
Are you saying your agent wants you to pay a selling commission of 3% NOW, even though the option to purchase hasn't yet been exercised? I assume the time frame for it to be exercised will be at the end of the rental term.
I would think a rental commission is due now...to be followed by the agreed-upon 3% sale commission when, and if, the sale takes place.
Personally, I wouldn't expect to be paid a sales commission out of the option money prior to the sale going through.
If your arrangement differs from that - it is between you and your agent, I would further assume you have an agreement in writing with the Broker that spells out how any commission (s) would be paid.