If you use the T100MD888+ PLC TBASIC language you can accomplish this quite easily by counting the number of ON alarm status. If more than 1 alarm comes on, (doesn't matter if it is 2 or 3), it will activate the MFT.
E.g. Assume the 3 alarm connects to inputs #1 to #3:
N = 0 ' N is the counter for number of alarm
FOR I = 0 TO 2
IF TESTBIT(INPUT[1],I)
N = N+1 ' increment alarm count
ENDIF
NEXT
IF N < 2 ' 0 or 1 alarm
RETURN ' do nothing.
ELSE
SETIO MFT ' activate MFT
ENDIF
That's all.