One thing I can say is try to have grace for the other child and the other parent. I am currently the parent of a 26 month old that behaves in undesirable ways by times - like not wanting to share with my 9 month old neice or even , embarassingly , pushing or stealing toys from my friends 10 month old. I feel judged by these parents , as though they are sitting waiting , watching for me to exact justice on my little guy for doing something that's developmentally normal. someday it may very well be your 3 year old having a hard time sharing or raising his voice to another small child.
As for how to deal with your child - just talk to him - even if he can't fully understand , stuff like ' sometimes its hard for other kids to share , why don't we find a different toy' - and help distract him.

It can be difficult when others don't parent the same as us, hence why many AP parents stick together

Are you close to this person? It's hard cause you want to make sure your child isn't hurt but not step on any toes. A gentle reminder to the older child would be acceptable to me.

As the parent of a 3 year old and a crawling baby I'd say they do need a constant eye on them even when they are siblings.

If this is a trend I would personally limit playdates.

That sounds hard to watch, an older kid yelling at your sweet toddler. I have seen that before with my own kids, Perhaps you can go over and suggest a better way for him to get what he needs. Say "Maybe next time you can ask him to move or ask his mommy to help you get him out of the way." A lot of older kids who are not exposed to young children do not have any skills for handling little folk. I tell my 4 1/2 year old a few things when playing with his 1 1/2 year old brother.

• If you don't want him to touch what you are playing with, put it on a high surface or play in another room.
• If you want something he has ASK HIM! If that doesn't work -possibly offer him something else. If he still wants to keep it move on to something else.
• If you feel angry or frustrated- ask me for help!

Most of the time that helps him navigate the playing field!
With someone else's older kid I think you can kindly suggest a different method for him to deal with your little one. I think it would be all right to ask him "Please don't yell at Jimmy, I think the yells were very loud. Can you think of a better way to play with blocks when we are together?"

Try to put yourself in the place of the other mother too. One day you will have 'the big kid!'