For q0=q1=q2=0, we replace these variables by 0 in the transition relation, but not those behind an X-operator. Hence, from

(q0 <-> !a | Xq0) & (q1 <-> a & Xq1) & (q2<-> q1 | q0 & Xq2)

we get

(0 <-> !a | Xq0) & (0 <-> a & Xq1) & (0<-> 0 | 0 & Xq2)

which is equivalent to