Don't go for another to "stop the issue" because I know a few 4dc families where #3 really struggles. The older two are older, responsible and #3 thinks they can never achieve their heights, and #4 is the baby which the older two dote on and gets away with everything.
I'm a middle child. Most of the time it didn't bother me much. Age ways almost exactly in the middle. I did find I tended to be lumped in with big sister as the "older ones" and "the baby" was so young they needed to have extra support, which was irritating.
eg. I remember when we got Trivial Pursuits for Christmas and #3 was asked questions ("no that's wrong, but I think that's too hard for you, let me look through and find an easier one and you can try again") or given hints until they got the answer right and I was expected to make do on one question, as per the rules. This made it a rather boring game as it could take 20 minutes for #3 to get one and then having got it right he'd get another go. When they eventually decided he wasn't being given another chance to finish his go he often had a strop, and got another chance...
This continued until #3 was far older than I was when we started playing. When I pointed this out I was told it was a game and I was being unsportsmanlike. I haven't played it since.
The things that used to really annoy me was with me and #1, my parents had a thing that if #1 had not got a big present (eg a watch) until a particular age, then I couldn't. So she got a watch at age 7, so I got a watch at age 7... and then it wasn't fair that #3 was the only one without one so they got it for their next birthday aged 4yo. I did try pointing out this was unfair and was told "but they really wanted one". Which as one of the reasons it had become a "big present" in my parents' eyes was because I'd begged and begged for one (and I'd never heard #3 ask at all) after #1 got one, seemed wrong.
The other thing was what I did, #3 had to do. I'm not sure why because me and #1 didn't do everything the same. This applies more to siblings rather than middle child though, But I felt that I couldn't be seen to excel at anything. If I was good, then it was expected that I was better than #3 so nothing should be said in case it put them off; if they were anything nearly as good it was commented on as amazing.
What I'd say is make sure that as much as you treat #1 as more responsible than #2, you should also treat #2 as that bit more responsible than #3.
Make sure they have different interests and hobbies. Not totally, it's fine to have some things the same, but ones where they can feel that it's their thing and they haven't either got #1 who has done it all (brilliantly) before so it's boring, or #3 who is snapping at their heels (because they're a genius).
Make sure #2 has time with you alone. It isn't always #1 & #2 or #2 & #3. Something my parents were good at. Tbf I was also quite good at creating those opportunities too.
The other one is hand-me-downs. Sometimes it's unavoidable. Sometimes they want them. We tended to have second-hand or homemade clothes anyway so it wasn't so much of an issue.
One of the issues I had as #2 was that I was small for my age and #1 and #3 were tall. So more than once I came home to find something that fitted me had been made over for #3 and I didn't fit (for some time) into the one #1 had grown out of.
Like a bike. Arriving home and finding my bike had had the saddle lowered and stabilizers put on when the next size bike, even at the lowest was way too big for me (and my bike could have gone up quite a bit) was a real slap in the face. Particularly when told that I was being mean because if they hadn't done that #3 wouldn't have had a bike. Had that a few times on different things.
I think with hand-me-downs it was worse because it was 2 girls then a boy. I felt I always had #1's things, and #1 and #3 always got new... at least new to them. From a parents' prospective, I can totally understand this. Financially it would have been silly to get me the same just for the sake of it, but it was something that upset me as a child.
It would have been nice if it could have at least been acknowledged. Perhaps "we'll get your bike a basket so some of it is new" type thing.
Otoh somethings that #1 and #3 got as birthday presents I just got handed, so I had that and a birthday present as well. So it isn't always a hardship!
I’ve heard my MIL say many times that her middle one (that I’m married to) is the easiest. He tells a different story…of being overlooked and left to himself precisely because he was easy and had no problems.
I'm in a similar position. I was and still am the best at compromise (personality rathe than learned), and that sort of thing, so it was always assumed I would back down. The other two would have thrown a strop if asked, whereas I often just said "no problem" for an easy life. Still happens today. 😂
It was acknowledged sometimes, but what I resented about that was it was always acknowledged in a "don't tell your siblings I've given you a mars bar for being kind" sort of way. I think normally I was so calm and accepting about it they didn't realise how much I minded. On the odd occasions I did say no, it was normally ignored too, probably on the basis that they knew I wouldn't strop as long or as loud.
But for all the things I've put, I don't think it was really that bad being the middle. I'm actually more independent than the other two, and I don't think that's a bad thing.
The fact you're aware that the middle child can have a rough deal, and so you'll look out for it, is a good thing. They may tell you when older it was terrible, but just as likely #1 will tell you how bad being the oldest (different set of issues) or #3 will complain about being "the baby" which is a whole new set of issues again.
Currently it's probably just "I'm not the baby" blues.