I think it's something that you have to be very careful about. I would suspect getting to the bottom of the reason will be much more effective than
I was very jealous of my younger brother. I was the middle child and it felt like he got away with everything and I was expected to behave like big dsis, but without the benefits.
Dm used to say "you've got to make allowances because he's going through a difficult time" when I complained about him.
I was nearly 20 when I turned round and pointed out he'd been going through a "difficult time" and allowances needed to be made for him for ten years. She stopped saying it for nearly a month after that. She's just stopped saying it about him about 3 years ago now. Roughly 30 years after she started.
I felt that because he was difficult and awkward I reaped the negative. And he was difficult (takes after my df), not just my opinion. So because he would make a scene if he didn't get something I had, he would get it too despite I'd been told to wait until I was old enough. Or because he would make more of a fuss dm would let him do something she'd just told me I couldn't.
If ever I objected I was told that I was mean and spoiling things. I'd often get "he'd lend it to you." (no he didn't, and he also tended to spoil/damage/never give back things I lent him) or "he'd do it for you" which often was irrelevant because I had to do it for him because he was younger and not able to.
What I would suggest is looking at how she may feel second best to him.
Make sure they do different activities. Not maybe entirely, but let her have ones that are her and she can be herself without feeling he's treading on her heels.
When you pull her up, don't reel off his virtues. She won't believe you, she'll just see it as your favouritism. Cries of "but he's so lovely" she will be hearing as "you aren't".
If she says something has upset her, listen. It may sound silly and you can't see why it would be upsetting. I can tell you some of the things that I found most hurtful sound totally stupid as an adult. However if I think about them, they can still bring tears to my eyes as it brings back the hurt.
Acknowledge she's hurt. Take note about what did upset her and decide how to handle any similar situations. It may be you agree with how you originally dealt with it, people aren't always rational. Ask a trusted friend without saying which child is which whether they agree with you.
Make time for her. I found recently my girls were jealous that I still read a bedtime story to ds. (9yo). I stopped with them around 5/6yo because they wanted to read to themselves. I didn't choose to stop, they didn't want me too-I would have continued if they'd wanted. But what they really wanted was time with me on their own so we do different things now.
Make sure he gives way to her at times. You may think he's always giving way to her. But does that mean you always come in on his side? IF you do, then she's thinking "if I don't stand my ground then dm will always make sure we do his way so I never get to do my way."
Don't fix every game for him to win, or give him hugely more chances. I can tell you playing trivial persuit with one child given questions (with hints) until he gets them right is boring. 3 years age gap is not that much in games, and there will come a point when he expects the extra goes. Choose games that are chance rather than skill if that's an issue. (my youngest is the best at Monopoly, and has been for some time)
If something is hers, then acknowledge it. Don't decide she's too old so it can be handed down or because he hasn't got one then she has to share it. If she knows that some things are hers and don't have to be shared then she'll be happier to share others.
Find something that is you and her special. Either he's too young, or just this is your special thing. Whether it's making a cake once a week, or going out for drink and a cake/breakfast once a month. Something which she knows is her special thing. And you don't stop it for anything. Find something else for him too.
Praise her. Tell her she's good at things. Let her feel you're proud of her.
It's not her nature.