I am not re-reading this whole thread right now, so if I am repeeating anything, I apologize.
Man is compared to the aitz hasadeh, the field tree, so the minhag arises from the comparison of the man to the tree, and just as the tree is not plucked of its orla until after three years, so, too does the child’s hair get cut after three years. In some cases, people have the minag on the third lag b’omer, but the ikkar is it is all minhag. There is no law to do it.
However,if you plan to follow this minhag, please undertstand in the case of someone wanting to “trim” the baby’s bangs because they are in his eyes, if someone has cut the bangs, then they have already cut the hair, so there is no point in growing the hair for three years, for the sake of an upsherin. it’s too late for that.