Maybe I did not phrase my question properly.

I am not taking that commission.  It is just that how to avoid the sellers' agent from getting 6% instead of 3% as they normally get. 

I have found out that if you state at the begining, it is feasible.  I am just not sure about the procedure, eg. where to put that on paper (black and white). 

Thanks for your response.   Really appreciate it.