@poorlyfoot congratulations! I’m around the same, due Feb 6th (with no 3!).
With my second, she was a bit younger (about 2.5) but we showed her the pictures and explained as much as we could - and got her a ‘big sister’ t-shirt which she loved! I remember her being super excited but she had no concept of time.
This time we’ll be telling a 7 and 4 year old and I think they’ll both be excited but haven’t really thought about how yet. We want to tell them asap after 12 week scan so there’s a bit of a gap before youngest starts school in September 😀
I’ve also been a whirlwind of emotions and definitely notice it’s worse when I’m shattered (which seems to be most of the time at the moment!) I think it has taken a while to sink in and get my head around it all.