I agree with cha-cha. I went back and read your questions from last October. And it's virtually the same thing: My son gets frustrated with my daughter's behavior; How do I change/fix my son?
Well, you don't. You stop expecting your son to just accept and tolerate annoying behavior from his sister. Just because he is older (and might be into reading books and capable of absorbing information about child development) does not mean that the way to approach their issues is to make him rise above and view things from a more mature perspective. He's 8. Even if he were 15, he shouldn't be expected to just "deal with" her annoying behavior.
SHE should be corrected about how to behave around other people. He should have the ability to remove himself from her presence when she is annoying (and let's face it, 3 year olds can be REALLY annoying, even if we love them) without any recriminations or guilt from you. He should have spaces he can *be* without her getting in his space and intruding into his 8-year-old-ness. That's the case even with kids closer in age. They should be able to have some expectation of privacy when they don't want to engage with others. Let him go read a book, without her interrupting his train of thought while he reads. Or ride his bike without her tagging along. Or play with friends without her under foot constantly and inserting herself into his play with friends. Or just sit on the grass outside and enjoy the sunlight without expecting him to listen to her endless questions and directions about how or what he should or shouldn't be doing.
(Now, I know that you haven't said she does all those things, but I'm using them as examples of the *types* of things that he should be able to do without constantly dealing with a 3 year old's intrusions. Every moment in life isn't a moment for siblings to bond.)
Yes, he should interact with her, regularly, and be kind, and include her in the things he can from time to time. But not always. Not every time. There are vast differences in their interests, abilities, and maturity. And when his patience is up, HE is not the parent who is obligated to watch her, or listen to her questions or whatever. That's your job. To listen, direct, correct, encourage, and so on. It's quite nice of an 8 year old to engage in those things with a 3 year old, but it is just that.. .being NICE. And it sounds like he has been expected to do this all of the time. You're all family, so I get that. But, he's 8 and not the parent.
I will add this, as well... if he happens to be an introvert, he may be even more frustrated than you imagine, and he might have been putting in far more effort than you are aware of (though it sounds like he's been quite patient with her)...