The specific RAID group listed in the error message (/aggr3/plex0/rg0) is the key to the message. In looking at the sysconfig -r output that you supplied, that RAID group has 15K 450GB disks in it and there are no 450GB 15K disks in your spares pool that are listed at the bottom of the sysconfig -r output that you posted here. If you do have spare 450GB 15K disks in the system, you may want to check the output of the disk show -n command to see if those disks are not assigned to be owned by this particular controller. If the spare 450GB disks are not assigned, you'll need to assign ownership to those disks with the disk assign command.
What's the FAS model for your controllers? Software disk ownership was introduced with the FAS3020/FAS3050 models and dates back to at least Data ONTAP 7.1, so the disk show command is available in Data ONTAP 7.3.3. What is the message you receive when you enter the disk show -n command?
Thanks for posting the sysconfig -a output. This helped clarify why the disk show command is not available. Your controller model is the FAS3020 and that controller model supported hardware or software disk ownership. By default, hardware disk ownership was used on the FAS3020 and that is your current configuration. The disk show command is only available when software disk ownership is used.
This then brings us back to you not having a 450GB 15K disk as a spare in this particular system. You need to add an additional 15K FC disk to eliminate the warning message.
That is an option. Doing so will drop the number of spares of your other disk types to 1. I would also check the value for the minimum number of spares option (options raid.min_spare_count) and set that value to 1 if it is currently set to 2 otherwise, you'll get warnings from Data ONTAP that you do not have enough spares for the disk type that only has 1 spare in the system.