Thinking about your problem I have found third simple way.
3) Use 2 relays to commutate phases (look at diagram):

F1 _______________________
F2 ______________         
F3 _____________ ________ _______TRANSFORMER
                 ^         ^
            RELAY2 OFF   RELAY1 OFF

relay 1 is powered from phase1 (diode+resistor);
relay 2 is powered from phase2 (same);
If phase 1 shuted down then relay 1 switches down to phase 2. If phase 2 failed then relay 2 switches to phase 3...
There are alot of tiny relays which do not increase your board alot.
Good days!