I have not worked out a full solution yet, but I thought I would share my thoughts so far.
First of all, I am interpreting a "Double-N set minus the doubles" of Dominoes as follows:
An individual domino can be represented as a pair of intergers x and y.-
-The set of Dominoes under consideration consists of all pairs of integers x and y such that: 0 <= x < y <= n for n an integer.
It is fairly easy to determine that the nth such set contains a number of Dominoes equal to the nth triangular number.
Since, as Jared said, the desired parition requires a set that is a multiple of three, this can only work for n such that the nth triangular number is a multiple three.
Taking the triangular numbers mod 3 gives the pattern 1, 0, 0, 1, 0, 0, 1, 0, 0 ..., meaning that n MUST be either a multiple of 3 or one less than a multiple of 3.
Also, since creating loops requires matching numbers in pairs, and each number appears n times in the sets under consideration, n must also be even.
Taking these in combination, the only values of n where such a partition might be possible are n that either a multiple of six or two more than a multiple of six.
This gives 2, 6, 8, 12, 14, 18, 20, 24, 26, and 30 as the first 10 values of n that might work.
Its fairly trivial to show that such a partition exists for n = 2, and I am assuming you have already found such a partition for n = 6. I am interested in seeing if n = 8 can produce such a partition without breaking the existing parition for n =6, so I am going to see if I can partition ranks 7 and 8 of the n = 8 set to meet the requirements.
Also, I have done some thought on partitioning sets of dominoes into longer loops, and discovered the following pattern for modding the triangular numbers by 4: 1, 3, 2, 2, 3, 1, 0, 0, 1, 3, 2, 2, 3, 1, 0, 0... , which gives possible values of n of multiples of 8. Modding triangular numbers by 5 gives the pattern 1, 3, 1, 0, 0, 1, 3, 1, 0, 0... or values of n equal to 10k or 10k +4 for integer k.
I will report back if I find a partition of the n = 9 set's ranks 7 and 8.
Just so you know, I am blind.
I pledge allegiance to the whole of humanity, and to the world in which we live: one people under the heavens, indivisible, with Liberty and Equality for all.
My Shapeways Shop