My answers first; then the explanation. You may still negotiate but if that is a reputable builder respecting the Realtor community (most builders get more than 80% of their buyers through Realtors), they WILL NOT hand over to you the 3% in-lieu of the Realtor commission. About an average range of discount, there is nothing like an average range. It DEPENDS.
Now, about having a Realtor on your side, it is not just for negotiation. A buyer agent is somebody who is on you side, to represent you throughout the process. That person is there while you do the home search, contract, walk through's with the builder, loan issues, closing statement review and final settlement at the attorney's office. In each of these stages, you can come across issues, sometimes issues you have never heard of. It can all happen. But an experienced buyer agent, using his/her education, knowledge and experience with each builder can fight for you and navigate you through, smoothly.
In addition to all the above, builders may be open to negotiation with an experienced agent (or even allow an agent even if he/she did not accompany the buyer at the first visit) because they know this person from past deals or he or she brings in a lot of customers.
In addition to all the above, some agents (like me) might be ready to share a part of their commission with you towards your closing costs etc. at settlement through HUD. That will be the icing on the cake.
Therefore, my suggestion is, don't think twice in getting a Buyer agent to help you through the buying process. You have nothing to lose but everything to gain.
Joe Matthews, Broker
Real Triangle Properties