I have a hodgepodge of drives in my array and would like to set it up with a 3tb parity and an array of 2tb drives.  I know i can replace a 2tb drive with a 3tb drive, but i have one 3tb drive in the array i would like to use for off line backup.  So i would i remove the 3tb from the array and replace it with a 2tb?  Running 5.0.5

Unraid will not allow you to reduce the size of a drive in an already assigned slot. In order to accomplish what you want, you will have to manually copy anything off of the 3TB that you want to keep, set a new configuration to erase the current slot assignments and assign the drives the way you want and recalculate parity. Be very careful that you know which drive is the parity drive, as you will lose any data on a drive if you assign it to the parity slot. I would recommend not assigning the parity drive slot until you have started the array once without it and verified that all the data drives are assigned the way you want them.