How do you get Sonic 3 and Knuckles on Classic Collection?

You can't get Sonic 3 & Knuckles on the Sega Genesis Classics Collection.

While the collection includes a lot of classic Sonic games, Sonic 3 & Knuckles is not part of it due to licensing issues. This is because the music for Sonic 3 & Knuckles uses samples from Michael Jackson's music, and there are complications regarding the rights to use those samples.

However, there are other ways to play Sonic 3 & Knuckles:

* Sonic Mania Plus: This game includes a remastered version of Sonic 3 & Knuckles called "Sonic & Knuckles."

* Steam, PlayStation Store, and other digital storefronts: Sonic 3 & Knuckles is available on several digital platforms.

* Emulators: You can play Sonic 3 & Knuckles on emulators.

I hope this helps!