Regret to hear about your problem.
In Real Estate, it is the contract that is followed. If your listing agreement spells out the commission and your signature is on that document then it is the amount on the signed paper that is owed.
Otherwise, written contracts would have no meaning. If a verbal agreement is reached in Real Estate
it is put on paper on an Addendum and signed by all parties.
When you signed the contract did you get a copy of the contract and what did that copy say?
If you do not have any commission spelled out in the contract that you have a copy of then, you should get a lawyer, and you will be pleasantly surprised and quite happy.
I would recommend that if you contract does indeed say 5% or even if it does say 6% that you have a conversation to the managing broker. Unfortunately, its very comment for agents to "cut commission" in order to gain listings but provide less than stellar service. In reality, most homes are not sold by the listing agent, but rather by another agent, therefore this agent was banking on the fact that she wouldn't sell this house and wouldn't lose anything by gaining your listings at a "reduced commission". She would win either way. If she sold it her herself, she gets full commission at 5%, if she didn't she still gets her 3% and you have to come up with the additional 1% for the buyers agent-- that is really where that money is going. Not to her.
I wouldn't focus on the fact that she personally didn't show the property in those 12 days that your home was on the market. The majority of the work for the Realtor is the paperwork and the marketing of the property to help get it sold. In 12 days she likely didn't have much of a chance to show it. Now if it had been 180 days on the market and she had never showed the property in that time, you would absolutely have a complaint there, but also, in that time, she would have likely use resources to help market the property (internet outlets, paper marketing, postcards, etc.) that she is hoping that by the sale of your home she will be reimbursed for from your commission.
Congratulations on the offer on your property and I hope the remainder of your sale goes smoothly.
I would talk with the managing broker for the listing broker's office and see what they will do for you. If you don't get anywhere with that, you could try contacting the real estate board and ask their advice.