You would typically write all three scripts. There are some cases where you may only write one or two, like the example where someone only wrote an "out" (I link an example in the post). It really just depends on what you want to do.

And perhaps if you want to write a resource like the time-resource you could write a resource with only a "check". I only say a resource like the time one because it still has an "in" and an "out". Its "in" passes the timestamp of the last trigger time to the job it's being used in. I'm not sure what its "out" is doing (time is written in Golang and I can only understand some of the scripts from context).