Great question Nikki.
I'm not sure what state you're in, but in GA dual agency is illegal unless written consent is given by both parties (as I think is also the case in most states). Dual agency is a very risky way to run her business and this agent is really opening herself up to some ethical issues... any ethical agent would have a real problem doing business this way (in my opinion).
In dual agency, the agent is not allowed to negotiate for the seller or the buyer. She has to remain neutral, and thus it's virtually impossible to have any negotiations. I'm not sure why anyone would ever agree to this unless it was a unique situation (for example, familial transaction). But otherwise, it doesn't seem to make sense for anyone.
If you aren't using a buyers agent, it's important to remember that this agent is representing the SELLER and will have a fiduciary duty to represent the SELLERS best interest, not yours. To her, you are just a customer and she has no loyalty to you.
I strongly recommend signing a buyer brokerage agreement with a buyers agent so you have someone in your corner to negotiate for you. There's really no reason not to do this considering that using a buyers agent is free for you (the seller usually pays all the commissions, although this can be negotiated).
Also, she has a legal duty to present all offers to her seller. She cannot legally exclude offers that give her 3% versus 6%. The seller is still paying the same 6% either way so this doesn't affect his/her net at the end of the day... so this shouldn't even be brought up by the agent when the offers are presented.
Hope this helps!