According to the above you removed device 3 not device 7....device 3
is missing...device 7 is still there and is a spare.

Thats right. 
 

Put what you though was 7 back

This device no longer exists.

...note that properly determining which
device is which is pretty difficult unless you have a commercial grade
wired enclosure from a tier 1 vendor that is actually labeled and
wired correctly (don't always count on that).

I thought, every device has its fixed device number in the raid:

mdadm -E /dev/sd[a-f]1 | grep Role
   Device Role : Active device 1
   Device Role : Active device 5
   Device Role : spare
   Device Role : Active device 0
   Device Role : Active device 4
   Device Role : Active device 2