斯 vs 这 vs 此 for the meaning "this"? What are the differences between these three, 斯 vs 这 vs 此, and any other characters or words for "this" in Mandarin Chinese?

I'm just a beginner and I've noticed at least these few characters are in my Memrise flashcard courses all with English translation of "this".
斯 is ancient Chinese language usage, and 此 is more used in writing language. 这 is most common nowadays.