I know this is a past post but i would like to add my observation of seed formation and germination to the record in regards to the three leafed cotyledon seedlings-
This is a mutation caused by imperfect pollination and seed formation. The three cotyledons had developed before germination. The mutation was caused by stress on the bud the seed came from. There is no trait as perfect conditions cause the DNA to produce no further mutated growth. Growers that report to breed this trait do so through high stress training or are just shit growers.
I see higher ratio of three leafed cotyledons and subsequent growth from seed of plants that received stress at the pollination and seed formation point. Observations made through over 500 germinations from different plants and the ratio of three leafed plants from different bud quality