Backup/remaking raid/restore from backup. Should work on any raid.
Some higher end raid controllers (eg. my 3ware 9650SE M24 is) are able to grow raid5/6 without data loss (took eternity to finish though), but no guarantee your is. Consult homepage of your raid controller vendor.
P.S. You might start getting issues with >2tb block devices (which your 3hdd raid5 after adding another disk will be larger then). Starting with need to use GPT instead of old dos partition tables, and maybe limited support in OS/bios.