Tree is 48 inches from top of soil line just acquired in July about 10 inches at that time. My question is I will be potting up from 3gal to 18-20 gal pot in coming year

i would just stick them in a bigger pot and leave the roots and soil alone. but that's me. i honestly don't think it would matter much either way. it's not like they'll be outside in direct sun after you prune the roots, so the plant would have plenty of time to regenerate roots. you could certainly argue that the plant would be better off regrowing a root system in the large pot.

seeing the things pot growers (pun intended) have to go through makes me very happy i'm just stickin' em in the ground :).

3 Gal to 20,no good,3 gal to 10 yes,that is good.
Root Prunning:Yes only the encircling roots at the surface ifr any.
You can keep the soil in,no problem,once you fill another 7,gal.,in top of the existing three.
Cutting encircling roots,is most important.
Best Regards

Hey there Sal,
i never had problem with plant going from 3 or 5 gallon to final pot size like a 20 to 30 gallon container.
I have done it in past along with just untangling roots from encircling each other then sticking in pot with rootball intact as much as possible and adding whatever soil i need. Just be carful when you water it so you dont drown and rot the roots if you choose that route.