consider xx1 being Element of X1, xx2 being Element of X2, xx3 being Element of X3 such that
A6:
x = [xx1,xx2,xx3]
by A1, Lm2;
take
xx2
; for x1, x2, x3 being set st x = [x1,x2,x3] holds
xx2 = x2
thus
for x1, x2, x3 being set st x = [x1,x2,x3] holds
xx2 = x2
by A6, Th28; verum