\(=\left(abc-ab-ac+a\right)-bc+b+c-1\)
\(=a\left(bc-b-c+1\right)-\left(bc-b-c+1\right)\)
\(=\left(a-1\right)\left(bc-b-c+1\right)\)
\(=\left(a-1\right)\left[b\left(c-1\right)-\left(c-1\right)\right]\)
\(=\left(a-1\right)\left(b-1\right)\left(c-1\right)\)