Meridian as yet don't recommend that 3TB (or larger) are used in the MD600, as they aren't as 'robust' as 2TB drives, and don't pass all the tests Meridian put them through.
However these larger drives are improving all the time, and Meridian only have a certain amount of resources to put towards analysing drives. So it's quite likely that there are 3TB drives being made, which would come up to the Meridian standard, if they had time to evaluate them.
In my experience we have several MD600 owners running 3TB drives, and they seem fine.
Nothing special needs to be configured, just put in a pair of identical 3TB drives and allow them to format.
My advice would still be use 2TB if you don't need the extra space, but if you need 3TB, and stick to 'reputable' manufacturers (I use Seagate) you should be OK.
Always make an additional backup though!
Rick