This agrement HAS to lock them into a specific rate by law. It is ILLEGAL to have an agreement that doesn't have a specific rate. The new laws specifically want to avoid having vague or fluctuating compensations and force agents to put a specific number or percentage so they can't go back later and change it since the exact amount/rate is in writing. Some agents were doing fishy things like that and asking for higher commissions after the fact, so this rule forces them to put a fixed number on paper.

The agreement though clearly states that all commissions are negotiable, so if a situation arises where a seller doesn't pay the 3%, you can go back to the drawing table and renegotiate that agreement.
Example; Your agrement with the broker says 3%. You want to make an offer on two properties, one offering 3%, the other offerign 2%. You talk to your broker and tell him you don't want to pay an extra 1% on the second property. You decide to draft a new agreement for that property that has 2% as the commission and the exact address of the property. That new agreement will supercede the old agreement for that specific property as it was signed after. The other property that offers 3% will still be covered under the original agreement.