You can't create any RAID with drives that are smaller than the existing volume so you're in a bit of a pickle as the 14TB will also have DSM and your configuration on it. First you need to decide which level of RAID you want (I'd go for RAID10 but you could go for RAID5).
https://kb.synology.com/en-global/DSM/help/DSM/StorageManager/storage_pool_what_is_raid?version=7 might help you decide. I think you'll need to backup your configuration then replace the 14TB with one or two of the 12TBs, initialise it/them, restore your configuration, add in the 14TB as a separate volume, copy the data across then nuke it. You can then add the third 12TB and add it and the 14TB to the RAID. Again,
https://kb.synology.com/en-global/DSM/help/DSM/StorageManager/storage_pool_expand_add_disk?version=7 might help. I
really wouldn't do this without a backup of the 14TB drive, just in case.