I've done pears and apples earlier and had 100% takes out of about 25 grafts. It's not going to hurt to wait though. You'll have a better chance of avoiding damaging frosts.
This is my 1st year topworking several year old trees. However, I have been tongue and whip grafting rootstocks that failed their graft the year before.
Not sure if it's frost. Almost think the scion just dries out and has a dead end before the sap is flowing. Either way, best to wait.
I wouldnt mess with a triple rootstock. Maybe pick the best 2 and graft, then cut off the lesser of the two, or the lesser desired one if both look ok.
Open center trees right from the ground rarely make it long term. Branch will likely break. IF it dont look right, prune.
What I do wonder with topworking. Is it better to prep the tree for topworking. Removing those long branches while dormant, then cut back a bit more and graft when the tree is ready.
Had 0 for 6 luck grafting northrup mulberry, kind f a rare zone 3 mulberry. Doing this again this year, but waiiting for leaf out. Apples are easy to graft, think pears are a touch easier.
Anybody clean or hydrate their scions before grafting. Mild bleach solution to kill mold. Sit them in a water for awhile to get water in them......