A true genetic triploid plant will have three nodes on the main stem as well as every branch.
A plant with a random three noded branch is just an anomally and not a genetic fixed trait.
These branches can revert back to 2 nodes at any time.
If you are wanting to try and preserve the triple noded branch(es), you will need to clone them directly.
Looking through your diary, all your seedlings started out as diploid plants (two nodes), if you had a true triploid, this would have been present right from day one, with three nodes/leaves, even three cotyledonds sometimes, appearing as soon as the seed started growing.
Hope this helps, Organoman.