I definitely think fall is a time when it will just put more energy into present buds, since it knows it will be sleeping and not have time to both make buds and energize them.
I call healthy nursery trees 100.
You took yours to about a 40, and it's come back to about 50-55.
I try to stay above an 80 for backbuds, well, anything actually, if we remain around 80 almost anything is possible and no design is lost....
..... if we "smartprune".
I'd want to see you back above 80 before cutting it again, maybe dipping to 60, but you'd want to be back to 80 before fall.
So since you gained 10-15 after one year, but with another slight trim, you can probably gain 20 points a year if left uncut. Which puts cutting again at about 2 years.
Much of this trouble comes from most of the literature being on, or for, finished trees. There isn't much literature on building things from nursery conifers. So folks either make cuts so drastic(far below 80) they never regain the energy needed for backbudding, or they make the constant exterior "pussyfooting" cuts, which doesn't send a strong enough signal in to get buds closer to the trunk.
Seems you've done both. Not to rag, but to let you know now you know so you don't have to give up on this. Of course, an after x-mas special on these could prove a good reason to start over! Though the tripleness of this unit is quite dope.
The trunks are decently different sizes and it stands proudly, so it's worth working out.
If there was a book, it would simply read, "build to cut, don't cut to build".
We can build entire future trees higher than 80, while sacrifice stuff has the whole tree at a 200. A 120 cutback is then safe. Build to Cut.
Cutting to build keeps us below 80 or worse.
On anything initially and in this one's 1.5 - 3 year future....
Depending on height, this is where I would make cuts, but not all at once of course, to stay above 80. For Taller at reds, for shorter at yellow. The idea is to remove as much growth as possible from far away, to send a large signal to what's lower to make buds. I reckon the closer you are to 100, the closer to directly after spring growth you could cut is, and never after the Summer Solstice.
Since your Largest trunk needs the largest first branch, it should be cut year one. Medium next, smallest last.
The catch 22 being, as soon as you cut the largest one, the smalls are going to catch up in size and you don't want that. Since you can't cut more off the smalls to compensate (-80), this must be planned for and prevented. If that means working on improving the size differential for a few years, well, taking longer never hurt anything!
Sorce