@kingsville grower (Mark) is the expert in this area. I have seen his 3x cuttings and they are very interesting. I don't know how many years he has been experimenting with them - i.e. if he has enough history to tell if the 3x cutting process yields better results down the road than the 1x cutting process.
But they certainly look more interesting as seedlings!
I did not respond initially because i have not been able to find written reference to the second and third cutting except to say it is done. I asked several professional growers and the only explanations i received were as follows.
1. Sometimes done again to improve results from first time.
Ie: shorter stem, better placement of spread.
2. Grower trying to create a new style.
The color change in the stem is not reliable timing as i have mentioned in several previous posts. The correct timing is when the second set of leaves have extended. Usually around 10 weeks after germination.
I have been doing the radial cut method for five years and have had good results. By that i mean the root spread and placement is better than average to begin with in the early stages however i believe it requires more labour and frequent management to prevent it from becoming unruly then the traditional root growth pattern. What it does do is develop a more compact root system faster for development of the tree using above ground methods.
I have recut seedlings a second time when i was transferring from the seedling flat to individual container. I did those that had very poor results after the first cut. The timing was not critical as the seedling and cuttings are all greenhouse housed and conditions controlled for growth cycle. The results were better the second time around for a few but not remarkably so. It is possible that the seedlings that did not respond well the first time around were not the strongest genetically. I have not bothered with a second cut on any of the subsequent crops.
For those who envision a radial root spread evenly distributed by the cutting process, the reality is not quite that clear cut. ( pun intended)
Pines readily produce roots above the cut. The roots still have a mind of their own with respect to direction.
Frequent repotting, trimming and rearranging is still required. i suggest a full bare root repot each year for the first three years to get the nebari in great shape for grow out. JBP are strong and vigorous as young trees and this is not a problem for them if done correctly. This process slows down the growth pattern but saves time in development over the long term. Using a colandar or pond basket helps but does not prevent root patterns that require correction.
I am currently repotting 154 four year old JBP that have been root pruned each year previously, as well as the container lifted and escaping roots cut part way through the growing season. These trees are requiring about 30 to 40 minutes work each this year to repot properly. A good reason to consider the number of seedlings you keep each year.
In summary i believe the radial root cutting can produce excellent results with attention to detail and frequent repotting. But then so can the traditional methods of airlayering, ground layering and root grafting. These are my observations based solely on the last five years of growing JBP with the radial cut method. I have attached a picture of one of the four year old JBP i recently repotted in a grow box.