Optimal way to save Social media posts from multiple platforms
You are asking how to implement inheritance on a relational database, and the 3 methods you mention are basically the 3 most common/basic ways to do that. See https://stackoverflow.com/questions/3579079/how-can-you-represent-inheritance-in-a-database for some discussion about them.

There is no "best" way to do it- it will all depend on how you intend to use your schema so it facilites your queries afterwards.