What happens when you, "Test out of 3 skills"?
Is that just another way of saying that you're taking a test? Or is there something more to it?
That option is there in case you already know a bit of the language and want to unlock more advanced parts of the tree immediately. Every x number of skills there is a checkpoint, you can get there by doing every skill or you can try to unlock every skill in one go, by having Duolingo test you on all of the skills that you haven't done between those that you have and the checkpoint of your choosing (you'll see there is more than one per tree). If you don't already know the language then those "checkpoints" are pretty much meaningless to you.
They are also useful in case someone loses their data and is forced to restart their tree. This way you can get back to were you left off after just a couple of tests (given you how to answer, that is).