You could use Part together with Span
In[1]:= list = {{a1, a2, a3, a4, a5, a6, a7, a8}, {b1, b2, b3, b4, b5,
b6, b7, b8}, {c1, c2, c3, c4, c5, c6, c7, c8}};
In[2]:= list1 = list[[All, 1 ;; 3]] // Flatten
Out[2]= {a1, a2, a3, b1, b2, b3, c1, c2, c3}
In[3]:= list2 = list[[All, 6 ;; 8]] // Flatten
Out[3]= {a6, a7, a8, b6, b7, b8, c6, c7, c8}
