Find the least node in a tree

I am working on Laravel + Vue and I've been stuck in the process of finding the least node in a tree.. This image you will see below is the structure. In one node there must be 3 sub-nodes if he got a 4th one, he, the new node will fill-in in the least sub-nodes below IMAGE HERE

I already implemented for the level 3 part but I am struggling on what should I do if ever the new node will fill-in in the least sub-nodes of level 4 nodes and level 5 up to level 7?

Anyone can share their knowledge to work make this work?

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