I'm the middle.
There's an aspect that you're aware, so you can make sure you don't do the typical:
"Oh you're old enough to do the washing up with the oldest, but the baby isn't." followed by "No, they're older than you so off to bed at the same time as the baby".
I remember feeling particularly irritated by being called along with my younger sibling as "the little ones." Fine when they're 2 and 6, but when you're 16, this is offensive to your dignity - especially in front of friends.
For me, I found that a lot of things I did, #1 had done it first (which was exciting) so it was no biggie for me. But then when #3 came to do it, they were the last, so a fuss was made. It did at times make me feel unimportant.
What I'd also recommend is making sure they do their own things. I found that what I did, #3 inevitably had to do. (not sure why, as I didn't do what #1 did, and #1 was both a girl and closer in age to me) And then you're feeling on your mettle the whole time. Because if you do much better, it's simply because you're older, and, hush, don't make too much about it because #3 will be upset; if you don't do much better then #3 must be brilliant at it, and you should be mature enough to be pleased for them, and praise them.
There's things that I gave up because I thought I was no good at that I have looked back as an adult and realised I was actually pretty reasonable at it, and #3 wasn't, but at the time it felt like they were brilliant and I was rubbish.
Part of it is personality. Both my siblings were probably ND, and with that quite awkward. So I was normally the one expected to compromise, and not mind. I rarely bothered saying I did, as it wasn't worth it.
But I chose to have 3 dc, so it obviously didn't bother me too much. I suspect #2 would tell you she has a really hard time of it, but actually she doesn't... She gets away with far more than she thinks she does.