3 year old wakes up in the morning in a horrible mood. Screaming uncontrollably

He does sleep through the night just fine

My daughter never did, my son has once. That sounds like it could be night terrors. I work in childcare and when I was in the toddler class there was 1 child that would get night terrors during nap time. So I looked it up to educate myself on what to do. It will seem like they're awake but they're not and you should not wake them up because it can make their crying/screaming worse because they'll be very disoriented and confused. All you can do is try to soothe them. I would rub her back and just keep telling her "you're okay, you're safe" and do my best to keep calm. The first time it happened I was terrified but then the other teacher was like "oh yeah, she has night terrors". When it would pass she'd continue sleeping like nothing happened and when she'd wake up she's fine and her normal self.

This is my son a lot but he's autistic and I've been giving him a little speaker and he calms down.

Is he hungry? My son is almost 3 and he wakes up a complete grump and feels better after breakfast or a little snack. I try not to engage with him much until after he eats lol

My 2nd born nephew did this & my SIL found he needed more water in the day & cut his milk or dairy to just morning & lunch. But he also had night wake 1x a night & would cry or scream so the doctor said it was night terror. She asked me for advice at the time I worked in special ed. So I suggested changing his food & diet to like, whole diet. So 1 or 2 small snacks a day but mainly healthy meals VS snacks. & pushing water even if she had to add a tiny bit of juice for flavor. She changed the milk to am wake or breakfast & lunch only & within 3 days he stopped waking in the night & stopped waking crying like you said. I almost forgot it happened to him until It came right back to me bc I'll never forget hearing him in the background of that phone call & my sil very distraught 😢💔💕💕💕💕💕

This is really common when waking up from naps (and probably other times too). it’s something to do with being disregulated and/or waking mid sleep cycle. Just stay close and reassure him that you’re there and be calm until he comes around, I find trying to snap them out of it or cheer them up or reason with them doesn’t work, nor does asking questions, it’s best to just be a calm presence

Maybe a little snack in his room can help, like a small bowl of cereal or crackers he can get for himself?

Oh it's like i had written this myself. I have similar age gap and share the room. 3 year old and 21 months. The younger one is all energetic happy and in the elder ones space in the morning to wake him up and cuddle him. Cute but my 3 year old hated it, woke up grumpy and screaming.
After a bit trial n error i found the answer. Change their bed time so that the elder one wakes up first naturally.

So now 3 year old is in bed by 6:40. In deep sleep by 7 max. Lil one in bed by 7:30. They sleep amazing now

Try tweaking your LO bed times, i think u'll be surprised