So, the content that you can bring in to 3 has to come from 2; owning 1 entitles you to the Legacy Pack for 2, ownership of which lets you bring those levels into 3, there's just that additional step in between. You'll have to, I believe, open Hitman 1 and you'll be able to redeem that legacy pack from the menu there. Once that's all installed, you should be able to download the GOTY Legacy Pass in 3 to play all those missions.
2 could be found on sale fairly regularly in the past, but I'm not sure what the case is currently.
Yes, this is all more confusing than it needs to be.