On Sunday, January 29, 2017 at 11:19:38 AM UTC-5, (PeteCresswell) wrote:
Drilled a 3/8" hole about 3" deep into the side of a maple tree - in
order to mount a screw eye.

Turned out to be the wrong location and now the tree has that hole in
it.

What would you do to minimize the insult?

I'm thinking just sterilize a 3/8" bolt and put that into the hole.

??
--
Pete Cresswell

Leave it alone.

It used to be common practice to tar over the wound caused by cutting
off a limb or to pour some sort hardening material in holes. This is
now considered not only unnecessary, but also probably harmful to the
tree.

Trees have natural mechanisms for healing themselves and preventing rot
and disease, especially from a simple wound like the one you described.
New wood has already started to grow and other internal processes have
already begun.

I quote from just one of the many websites found on the ole' interweb.
Granted, they are discussing those large holes that you often see in
tree trunks, but the same advice holds true for a small hole such as yours:

"Is Filling Holes in Tree Trunks a Good Idea?

In the past, it was often recommended that filling holes in tree trunks
was a good way to correct the tree hole. Most tree experts now agree
that this advice was incorrect. Filling holes in trees causes problems
for several reasons. The material that you fill the tree hole with will
not react to the weather in the same way the tree wood will. The material
you use will expand and contract at a different rate, which will either
cause more damage to the tree or can create gaps where water (which leads
to more rot) and disease can get trapped. Not only that, but if the tree
must be removed at a later date, fill materials can create dangerous
situations to the person removing the tree. Imagine if someone using a
chainsaw were to hit a concrete fill that they were not aware of in the
tree."

....or in your case, "Imagine if someone using a chainsaw were to hit a
3/8" bolt that they were not aware of in the tree."