Time Machine: Including 2 External Drives

How can include 2 External Drives (Mac: 500GB; Ext1: 500Gb, Ext2: 2TB) in the Time Machine backup (to a third disk: 5 TB).

The Apple documentation says:

"To stop excluding an item, such as an external hard drive, select the item and click the remove (–) button."

I need to include two external drives.

But the "stop excluding" option, i.e. remove(-) button works only for one of them.

(It's disabled after I chose the first one.)