From memory SEC 3 just controls a couple of ground spoilers (might also have a feed to the THS) but i'll check on this for you.
The SFCC's are used to move and monitor the slats and flaps only. SFCC 1 operates using the Blue hydraulics and SFCC 2 operates using the Green system.
If either computer is u/s or one of these two hydraulic systems is off or u/s the flaps/slats move at half speed.
That's the simple operation from memory but if you need more detailled info I can certainly dig it out for you.
The aircraft can be flown with all the SECs, ELACs and SFCC's u/s if reqd using the mechanical backup of the THS and rudder. I've seen the A320 simulator flown well under these conditions and it was controllable.
Ideally you need one SEC (No 1 or 2) and one ELAC to remain in "electronic" flight control.
As a footnote, all i've described above is extremely simplified. Each EFCS computer has a command and monitor channel and the SFCC's monitor each other so it does become very complicated to explain
