Yes. It's already in 4.2.16 (currently in public beta: http://readynas.com/forum/viewtopic.php?f=51&t=48457
So you'd update to 4.2.16 first (requires reboot) then add the 3TB disks (one at a time of course).
Assuming you're using X-RAID2 you'd need two 3TB disks to expand your volume (expansion can only take place when redundant space can be added). You'd add one 3TB disk, wait for the resync to complete then add the next disk.
It doesn't matter which disk you replace, but obviously you'd want to replace a failing/dead disk if there is one. For this reason it's recommend that you run the "Test Disks" boot option using the boot menu: http://home.bott.ca/webserver/?p=252
If the disks all pass the tests then you should take a look at the SMART+ stats under Status > Health in Frontview and replace the disk that you think is in the worst condition.
You should make sure you have an up to date backup of important data primarily stored on the ReadyNAS before upgrading to higher capacity disks (you should backup important data regularly anyway). When you remove a disk you remove the redundancy so there is a period where you are vulnerable to a disk failure while the new disk is being synced into your array.