eh?
3! accounts for all possible perms to put (a,a) , c , d
theres only 3 objects. Don't times it by 2! for the 2 a's coz they're identical. You actually times it be 2!/2!.
If you do it your way, there 3 positions for AA block, 2 positions for C and remanding 1 for D. 3*2 = 6