I'll change ingredient A to 1/2 instead of your 1/3; easier to analyse; all you need is:
We complete a "drum" this way:
Ingredient A = we use 1/2 of a bucket
Ingredient B = we use 1/4 of a bucket
Ingredient C = we use 1/8 of a bucket
Make the buckets 8 units each.
To make a "drum", you need:
4 units (1/2) from A, 2 units (1/4) from B and 1 unit (1/8) from C;
so drum is size 7 units; that's it...no need to worry about size and water!
One C bucket accomodates 8 drums; to complete, 2 B buckets and 4 A buckets are required.
So 7 buckets are required for 8 drums.
If you can carry say 70 buckets, then you need 10 C's, 20 B's and 40 A's;
and 80 drums will be accomodated.
Use same method to analyse your 1/3 A and 64 buckets.