This trident maple featured in this post was grown from 3 separate seedlings. They were planted close and held together until the separate trunks grafted themselves into a single trunk at the base. Multi trunk bonsai are not particularly common because every extra trunk adds complexity and the possibility of faults that would make the whole tree unattractive.
This particular triple trunk has some desirable features: The trunks harmonize with each other; The 3 trunks are of varying heights; The trunks divide low on the tree; Branching alternates on all 3 trunks.
Aesthetically it is desirable for the tallest trunk to also be the thickest and the thickness of other trunks should be proportional to their height. Originally the far right trunk was markedly thicker than the taller, centre trunk but I am gradually changing the relative thicknesses.
Achieving this has been a slow process and involves slowing the growth of the shorter trunks while promoting growth on the taller one to encourage it to thicken a bit quicker. Defoliation is one technique I have been using to hold back growth in the smaller trunks. Here’s a photo after defoliating both the shorter trunks.
Defoliation consists of cutting the leaves off a tree. Cut through the leaf stem (petiole) leaving most of the petiole attached to the tree. In a few weeks new buds will form at the base of each petiole and the remaining petioles will drop off as the new buds open into new shoots.
Defoliation can help to increase the number of twigs on a tree. A side effect is that there will be more leaves and each leaf will be smaller than before but removing leaves is quite stressful for the tree so should only be used on healthy, well fed specimens.
You can defoliate an entire tree to increase the density of shoots or partially defoliate as I have to retard growth in some areas and encourage others
If your tree is still developing defoliating will only slow the growth rate and it will take longer to achieve the size and thickness you are aiming for.