Firstly, a 3 mill dummy is generally considered to have too little health to get a good reading.
You dont need to worry about good sustain and builds that ramp damage will have lower readings (literally any build that uses relequens, for example)
On to 6 mill, these where the goto since they where added pretty much, good dps is generally anything above 30k, but given that a dummy just stands there for you to patchwork on you should be hitting 35-40 dps in a good build, 45-50k this PTS cycle on a highly tuned build.
Its common also for people to take healers with parsing, to feed them prayer and orbs, and keep fracture/breach up on the dummy. These builds will hit more in the range of 60-80k (tuned builds). This is sometimes thought of as cheating, but was also a necessity for fare class to class comparison for builds that dont have fracture and breech in their rotation.
Recently a raid dummy "Iron Attro" was added, this provides every raid buff and has every debuff you can hope to get in a well rounded raid party. Tuned builds on these hit from 70-100k.