It seems that Vector3Int (alongside other int type vectors) were implemented on Unity in version 2017.2.0 (12 Oct, 2017). Still, i’m using version 5.6.5 (28 Dec, 2017) and for some reason i can’t use the native Vector3Int type, that should be included on Unity.Engine.
Can anyone help me?
Uhm haven’t you just answered your own question? Since you use an older version of Unity this type doesn’t exist in your version. You have to download the newer version if you want to use it. As alternative you can of course implement your own version. Some time ago i’ve written the Vector3i struct which provides almost the same functionality. I also still use Unity 5.6.1f1 so i can’t use the Vector3Int struct as well.
There has been several newer versions since 5.6.5, just check the drop down menu at the top right. Also you use a patch release. Patch releases are intermediate versions which do not belong to the normal update chain.
Vector3Int was implemented in Unity 2017.2.0 as you’ve stated. You are using an older version, Unity 5.6.5. That is why it does not exist. As Bunny83 mentioned, there are many versions after 5.6.5. If you really need it you can update to the newest version but I’d only do so if that newer version has features related to your game…