I add 3 disks to root aggr from system manager by mistake. There is no data in it. Is it possible to remove those disks (free those disks from root aggr)? If yes, how?
No, it is not possible to do so.
You can move the root volume non disruptive with a cf takeover/giveback tho, so if you have at least 2 spare disks, you can "rescue" yourself.