Sleengster 3 STeam game - how to get it now?

It's not only delisted, but also banned. Giving it away in a Steamgifts GA would lead to an issue, since the API can't detect it anymore. Thus the winner would have a non activated win.

The third part wasn't bundled, before it got delisted, so it's more difficult to get it. You might want to check Steamtrades, though searching for it will probably also return results for Sleengster and Sleengster 2.