I just found out why my third network card didn't get set. It's a typo in the examples of the interfaces file.
For Step 3 and step 4 the word adress is written with one d while it should be 2.
# Step 3 (optionnal): configure DMZ
# Default: eth2 / fixed IP = 192.168.1.100
#auto eth2
#iface eth2 inet static
#
#
#
#
# Step 4 (optionnal): configure bridge
#auto br0
#iface br0 inet static
#
#
#
#
#
#