Doing a DoN with 3 people when one or even two are around lvl 60 is easy because the average level of the group is less then 70. Even with two lvl 70 and a lvl 60, you get a DoN with an average mob level around 65. Most lvl 70 players with time+ gear can just cut through the mobs and lvl 60 will scope the experience. I expect it would work as well or better if you had only one level 70 and two level 60-63.
You might try a DoN with high numbers of mobs to maximize the experience points. We mostly did "clearing the drakes", which seemed like more experience/hour then "creator".
Andras
