They're all fine like Gary says. Very difficult to explain when one version is more appropriate than another. For me, they are completely interchangeable *most* times. It just comes down to speaking style or habit which version you use. I *think* (only my opinion) that you tend to use "There is no point IN doing something" when you are talking about the "process" of something, rather then simply that thing. As an example, "There is no use IN doing homework" means you are thinking about the process of homework, about the time it takes. There is a time element in your mind. In contrast, when we say "There is no use doing homework" this time element or process is not stated. But like I mentioned, this is only my opinion. Other native speakers may have a better explanation or disagree with me.

The 'in' seems to be optional. There's no point (in) + gerund It's no use (in) + gerund Personally, I wouldn't use 'in'. "No use crying over spilt milk" and "No point crying over spilt milk" both sound fine and are probably more common. But do take notice of what Gary said about 'sth' or 'smth'. This is a dictionary abbreviation, like writing 'n' for 'noun' or 'pres perf' for 'present perfect'. It is NOT A WORD! If you write 'sth' in a normal sentence, you are likely to a) confuse most people. To us, it looks like a mistake. b) give yourself away as coming from eastern Europe or a Slavic country. You're generally the only folks do this.

They are all fine. I don't know if there is a grammar rule or not, sorry - I only know what sounds natural. 'sth' is not an abbreviation normal people use. It is probably better to avoid using it.