How can we prepare our 3-year-old grandson for visits with his late-stage Alzheimer's-afflicted grandmother who irrationally scolds him?

No offense, but there's absolutely no reason to take a three-year-old to see someone like that. He won't remember her, and if he does, they won't be good memories.

A 3 year old child should not have to be exposed to an Alzheimer's patient who is 'irrationally' dealing with his normal 3 year old behavior. It serves no useful purpose for you to be trying to control her behavior OR his behavior. It makes better sense to show him grandma over Facetime and put the tablet down if grandma starts getting upset, if you insist on a 'visit'. Better he should have no memories of the woman than ugly ones. All stories don't have happy endings, unfortunately, and at 3 years old, your grandson doesn't need to be traumatized needlessly. I have grandchildren myself and never brought them to see my mother who had advanced dementia b/c who knows WHAT words would have come out of her mouth?? My grandchildren can look at photos of their smiling great grandmother in her younger days before the ugly disease of dementia turned her into someone else entirely.

I would rather protect the young innocent kid than the old demented grandmother. A 3 y.o. child is too vulnerable emotionally and incapable to comprehend what dementia does to people.

Don't take the three-year-old to visit. If he actually does have a memory of his great-grandmother at some point in his life, it will be of a terrifying old woman.
Don't make him be around his great-grandmother with Alzheimer's and don't force her to have to be around a three-year-old who you cannot control. Neither party benefits from those visits.
Leave him home. He does not need to make memories with hs great-grandmother and she will not remember spending time with him.
I have a few memories when I was a little kid of visiting one of my grandmother's elderly aunts whose son brought her to America.
Let me paint you a picture of this experience.
Think of a toothless corpse with long, dirty nails wearing a black dress, tied in a wheelchair that screams in Italian. She also burped and farted continually and smelled absolutely disgusting.
Now picture a little kid and a bunch of adults who've had a few glasses of the homemade vino because it's a holiday, and they make the little kid go and "Give zia (aunt) a kiss". 'Zia' grabbed ahold of my hair with her her clawed hand and nearly pulled it from the roots.
I'm almost 50 years old yet I still remember the terror 'zia' Lucrezia
like it was yesterday.
Don't do that to a little kid. Leave the three-year-old at home.

I am 75 and confused by my step-mother's behavior. She has FTD.

Me, I wouldn't take a 3 yo to visit someone who has late stage dementia. The child does not need to be exposed to that behavior.

I would not expose my small grandchildren to an Alzheimer's grandparent.

I would seriously limit his visits. At three he is incapable of your understanding really even the simplest explanations. But you can TRY. You could tell him that Gran has an illness that makes her very cranky, and that she isn't always remembering things, and so on. Tell him that Gran may seem cranky, but because there's not much that can be done for this illness, everyone must try to be as kind as they can.
KEEP VISIT VERY SHORT when you have the young one with you
There is no real reason for such a young child to have to be exposed to this overlong. He is very unlikely to remember or care a lot in future.
Give him fun things to give to gran. Help him make a card with her as a young woman, better still, a child, or as his gran making things for him. Teach him to say "I am so sorry you don't feel good, Gran, and I hope you will be better soon". Take it slow, gently, and keep it short. Use it as a teaching moment; we all need to learn compassion as early as we can.
Hope you'll update us.

While the idea of visiting is wonderful (cue ethereal music and unicorns dancing)
It is sometimes not practical.
I used to explain my Husbands dementia and the way he acted in stores (he would make noises that to kids sounded like crying) by comparing it to kids they might know in school that have Autism. I tell them his brain does not work like yours or mine.
You can explain some things to kids but 3 is a bit young to grasp that concept. Then they may also fear that their brain can get broken somehow.

Talk about grandma. Tell him she is not well. but tell him stories and show pictures of her. Tell him what she used to be like. He can know her through stories,.

I'm with the group here you shouldn't take your 3 year old to visit. Your grandmother won't remember the visit and your grandson won't have a pleasant visit.

Don’t take child to visit agitated dementia/Alz grandparents. It will be a disaster & it’s child abuse. Granny won’t remember visit 5 min later..but impressionable child will remember being abused forever. Don’t do it!!!