Yes, it will only move the data from disk 2 if a balancing rule causes it to be moved (if you have disk space equalisation turned on, for example). Otherwise, it will stay put.
 You could just set the drive-overfill plugin to 75-80%? Then if any disk reaches that capacity, it'll move files out? Personally, I assign a pair of landing disks for my pools. Two cheap SSDs where incoming files get dumped. DP then moves them out later, or if it fills up. Note that the disks should be larger than the largest single file you would put on the pool.
If cost is an issue, you could try the following setup with existing hardware instead:
Install the SSD optimizer plugin
Tell DP/SSD Opt. that Disk 2 is an SSD and un-tick the "archive" setting
Make Disk 1 and 3 "Archive" drives
Change your file placement rules so that only unduplicated files go on Disk 2, and only duplicated files go on 1 and 3
That way, all new incoming files get put on "Disk 2", then later when your duplication/balancing rules engage, it will move the data off of Disk 2 entirely, and duplicate to 1 and 3.
This assumes you do not have "real-time duplication" enabled. If you still need the total capacity of the 3 disks, then perhaps a small investment in a 120/240GB SSD to use as a landing drive might be a good idea, and substitute "Disk 2" with "SSD/Disk 4" in the above setup?