If you really need to define it I'd say Triple trunk would be a subset of Clump style. Obviously triple can only have 3 trunks while clump can have many.
Important guidelines: 3 trunks divide near the base. Each trunk a different height. Tallest trunk is the thickest, shortest is thinnest.
Less important considerations: Lowest branch is usually first branch of the smaller trunk. Branches will usually follow the same guidelines as single trunk tree ie lowest branch to one side and forward, next branch from back or opposite side but a bit higher but in triple the branches come from all the trunks (that's not really clear so eg. Lowest branch from smaller trunk on right side. 2nd branch from a bit higher on left trunk. 3rd branch from back of centre trunk 4th branch from right trunk, etc. note the apex of one or both of the smaller trunks may fill the function of a 'branch' as it will lean out away from the main trunk.
Here's a couple of mine that will probably show better what I mean.
Trident 4 2010 05 a.JPG
Trident triple 2010 07 a.JPG
You can see what I mean about the apex of the left trunk taking the place of a branch in the second pic. I am considering taking the top off the smaller trunk to leave a better negative space between it and the branches above from the main trunk.
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